CHAPTER XIX. THE HABITS AND LIFE-HISTORIES OF COMMON INSECTS.

62. THE COCKROACH: A TYPICAL INSECT.

1. =Habits.=—In what places have you seen cockroaches? Are they often to be seen during the day, or do they, in general, come forth only at night? What is the colour of the body? Put a live cockroach under a tumbler, and watch its movements. In what position is the head held? Notice the long _feelers_; how are they used? Look at the lower part of the head, and try to see the _palps_, which resemble small feelers. How many legs has the cockroach? Watch the rhythmical movement of the hinder part of the body. Soak a small piece of bread in milk or water, and put it under the tumbler; watch the cockroach feed. How does it use the palps? Notice that the jaws move from side to side. How does the insect clean itself?

2. =External characters.=—Put a cockroach into a test-tube, and dip the tube into boiling water; this kills the insect instantly. Notice the thin “shell” which covers the outside of the animal. Make out that the body consists of (_a_) _head_, carrying the eyes, feelers, and jaws; (_b_) _thorax_, carrying the wings and legs; and (_c_) _abdomen_. Examine them in turn.

(_a_) _The head._—Notice the vertical position, the black, kidney-shaped _eyes_, the _feelers_, the “upper” lip (_labrum_), and the small, blackish _mandibles_ at the sides of the labrum. Pass your knife-point close behind the labrum and into the mouth, and see the “lower” lip (_labium_), which lies behind the knife, _i.e._ behind the mouth. Cut off the head and carefully strip off the labium. Put it on a sheet of paper and examine it with a lens, comparing it with Fig. 222 (_Mx._ 2). Examine the same head from behind with a lens and see the pair of _first maxillae_. Notice how they are attached to the head, and then remove one and compare it with Fig. 222 (_Mx._ 1). Work one of the mandibles backwards and forwards with the point of a pin, and then remove it and examine it with a lens to see the toothed inner margin.

(_b_) _The thorax._—Notice the overlapping fore-wings (_wing-covers_). Pull them aside with forceps; notice that they are narrow and rather stiff, make out their points of attachment at the corners of the second segment of the thorax, and then cut them off with scissors. Pull out the delicate _hind-wings_ in the same way, carefully noticing their fanlike method of folding, and their points of attachment to the corners of the third segment of the thorax. Stretch one out, cover it with a piece of glass to flatten it, and then draw it, marking the lines of folding. (Notice that some cockroaches of the common species found in kitchens are destitute of wings, and have only very small wing-covers. These are females.) After removal of the wing-covers and wings, the three segments of the thorax are very distinct. Notice that each segment bears a pair of _legs_. Take off one of the legs and draw it. How many joints has it? What is the use of its bristles?

(_c_) _The abdomen._—Observe the line which runs along the dorsal middle line of the abdomen; this marks the position of the _heart_. Examine the method of telescoping of the segments of the abdomen, and the soft membrane which connects the dorsal and ventral plates of each segment. Notice that the abdomen bears neither wings nor legs. Observe the pair of short palp-like bodies (_cer._, Fig. 223) at the end of the abdomen, and between them, in the male, a pair of more slender styles. In the female observe the boat-shaped “_brood-chamber_” (Fig. 223, _st._ 7), on the ventral surface.

(_d_) _The spiracles._—In the thin membrane between the dorsal and ventral plates, at the junction of two abdominal segments, look for a small hole (spiracle) leading into the interior of the body. How many abdominal spiracles can you find on each side? Observe the two larger spiracles on each side of the thorax, between the first and second, and the second and third legs.

=What is an insect.=—The word insect is so commonly applied to animals having no claim whatever to the title, that it is advisable to point out at once some of the features which distinguish insects from other animals with which they are often confused. Insects, spiders, crustaceans, centipedes, and their near relatives, all have jointed bodies and legs, which are covered by a continuous suit of armour of a substance called =chitin=. In some cases, as in lobsters and crabs, this is for the most part hardened by mineral matter to form a stout shell, being soft and flexible only at the joints where ease of movement is required. In other cases the layer of chitin may remain thin and delicate, and all gradations between the two extremes may be found. Though soft where movement of one part on another takes place, the chitin is always firm enough, elsewhere, not only to form a protection for internal organs, but also to afford attachment to the muscles which move the body and limbs. It is therefore a =skeleton=, but as it is on the outside of the body it is called an _exoskeleton_, to distinguish it from the internal skeleton (p. 224) of the vertebrata. Animals such as insects, spiders, centipedes, and crustaceans, which have jointed bodies and legs, and are covered by a chitinous exoskeleton, are called =arthropods=. Insects may be at once distinguished from all other arthropods by the _single_ pair of feelers and the _six_ legs. Many of them, but not all, possess wings—structures which are found in no other arthropods. Insects are always air-breathers when adult.

=The cockroach.=—The cockroach is not a general favourite, but it displays so well the essential features of insect structure that it affords an excellent introduction to the study of the more popular members of the class, which in many respects are highly specialised. It is also easily obtained, and of fairly large size.

The body of the cockroach (Fig. 220) is very distinctly divided into three regions: (1) the head (Fig. 221) which carries the feelers, the eyes (_ey._) and the jaws (_man._, _max.¹_, and _max.²_); (2) the thorax, separated from the head by the slender neck, and bearing the legs and the wings; and (3) the abdomen, which bears neither jaws nor limbs.

The =head= (Fig. 221) hangs nearly vertically from the neck. The large, compound =eyes= (_ey._) are somewhat kidney-shaped; the long, jointed =feelers= are set in sockets just below the eyes. The front of the head is smooth and rounded; hinged on its lower edge is a flap which hangs down in front of the mouth, and is called the _labrum_ or upper lip. Just behind the labrum, and attached to the side and front plates of the head, is the first pair of =jaws=, the _mandibles_ (_man._, Fig. 221; _Mn._, Fig. 222); they work from side to side, and their inner edges, which bite against each other, are strongly toothed. The second pair of jaws, called the _first pair of maxillae_ (_max.¹_, Fig. 221; _Mx._ 1, Fig. 222) is situated behind the mandibles. Each first maxilla consists of a two-jointed base and two branches; the inner branch bites against the inner branch of the first maxilla of the other side, while the outer and longer branch forms the _maxillary palp_, which acts as a small feeler. The two _second maxillae_ (the third pair of jaws) are broadly like the first maxillae, but are partially fused to form the _labium_ or lower lip (Fig. 222, _Mx._ 2), which hangs down behind the mouth. The outer branches of the second maxillae are called the _labial palps_ (_Lab. Pa._). It is well to acquire clear notions of the arrangement of the jaws in the cockroach, in order to understand the great modification which the mouth-parts of many other insects have undergone. It is important to notice that the jaws of all arthropods work from side to side, not vertically as do the jaws of vertebrates.

The =thorax= consists of three segments, each of which bears a pair of =legs=, upon which the weight of the resting or walking insect is supported. It has been found, by instantaneous photography, that in a walking insect the weight is carried at any instant by the first and third legs of one side and the second leg of the other side. At the next step the body is carried by the remaining three legs. In the American (Fig. 220) and German cockroaches both sexes possess two pairs of =wings=, which are fixed at the front angles of the second and third segments of the thorax. The hind-wings, which alone are used in flight, are folded up fanwise when not in use, and are covered by the smaller fore-wings, which are generally called the =wing-covers=. In the common cockroach of this country, only the male has well developed wing-covers and wings. In the female the wing-covers remain small, while the wings themselves have disappeared.

The =abdomen= consists of ten segments, although only eight can be clearly seen without dissection. The “telescopic” arrangement of the segments is well shown in Fig. 223. Where one segment joins the next, the chitin remains thin and flexible. In each segment the chitin forms a dorsal (p. 217) and a ventral plate, which are joined together at the sides by a flexible membrane. The chitinous covering of the upper surface of the abdomen is so transparent that the =heart=, a median tube, may be seen through it.

=How an insect breathes.=—If the sides of the abdominal segments be carefully examined, a small aperture will be seen perforating the thin layer of chitin between the dorsal and ventral plates, at the point where each segment joins the next; and, in the thorax, larger but similar openings will be found on each side between the first and second and the second and third legs. These holes are called =spiracles= (Fig. 223, _spir._); they lead into a complex system of air-tubes which ramify throughout the whole system and supply the organs with oxygen. The tubes are prevented from collapsing by a spiral lining thread of chitin. This peculiar method of respiration, which is characteristic of insects, should be carefully contrasted with the manner of breathing in a rabbit or a man (p. 242) and in a tadpole (p. 344). It ought to be borne in mind, however, that the essence of respiration is the same in all living things, and consists in a replacement of excess carbon dioxide by fresh oxygen (p. 242); the difference lies merely in the manner of effecting the exchange. In the rabbit, the blood, carrying the excess of carbon dioxide, is brought to the air (in the lungs); in the tadpole it is exposed, in the gills, to the dissolved air of the surrounding water; while in the cockroach the air is carried directly to the tissues needing fresh oxygen. The air of the tubes is renewed by a rhythmic action of the abdomen, which can readily be observed in the living insect.

=The internal organs.=—Figs. 163 and 223, which represent a general dissection of a frog and of a cockroach respectively, exhibit in the most striking manner the essential differences in the structure of vertebrates and arthropods. The fact that the skeleton of the frog is wholly internal and that of the cockroach wholly external, has already been mentioned. It will be seen, further, that while in the frog the central nervous system (the brain and spinal cord) lies entirely dorsal to the digestive canal, in the cockroach the great nerve chain (Fig. 223, _n_, _n_,) is mainly ventral—the only dorsal part of the central nervous system being the so called brain (_brn._), which is connected with the ventral chain by a ring surrounding the gullet (_gul._). The heart of the frog is ventral to the digestive canal; that of the cockroach is the most dorsally placed organ of the body.

=Habits and life-history.=—Cockroaches infest kitchens and pantries; they are of social habits, and hide together in crevices during the day, but come forth at night to feed. They are not at all fastidious as to diet, but are especially fond of starchy foods, which they are able to digest (p. 234) by means of the fluid formed in their large salivary glands (Fig. 223, _sal. gl._). The eggs are laid sixteen at a time in a little oblong case, which the female carries about in a boat-shaped receptacle (_st._ 7, Fig. 223) at the end of her abdomen, until she finds a suitable place in which to deposit it. When the little cockroaches hatch, they are quite white, but except for the absence of wings they closely resemble the parents in form, and run about and feed freely from the first. The chitinous exoskeleton is shed from time to time as the animal increases in size, a new coat, at first soft and wrinkled, but rapidly stretching and hardening, having previously formed beneath the old one. Shortly before the last moult, the wing-covers and wings begin to grow out from the angles of the second and third thoracic segments. In this manner the young animal gradually takes on the form and dimensions of the adult.

=The position of the cockroach in the insect class.=—The cockroach is familiarly known as the “black-beetle,” but the name is a very misleading one, because the true beetles differ from cockroaches not only in structure, but also in life history. Cockroaches are grouped with earwigs, grasshoppers, crickets, and locusts in the order =Orthoptera=,[28] a term alluding to the fanlike folding of the hind-wings.

63. A WATER BEETLE.[29]

1. =The habits of Dytiscus.=—Search a pond for water beetles. Put them in a wide-mouthed bottle with some of the weeds to be found in the pond, and take them home and observe their habits. Among the larger beetles—especially from ponds with a clear surface (not covered with duckweed, etc.)—will probably be seen some with a yellow band round the edge of the upper surface. These are _Dytisci_. Notice the ovoid, smooth body, the breadth of the hind legs, and—in the male—the cup on each fore leg. Try to see how the legs of each pair are used. On what does the animal feed? Take a small piece of meat in a pair of forceps and hold it near the beetle’s head. If the animal does not notice it, stroke one of the feelers with the meat; how does the beetle behave? What is the use of the feelers? Notice how the beetle rises to the surface immediately it stops paddling. Which end of the body sticks out of the water? Why does the beetle need to come to the surface? In April look for larvae (Fig. 224, _A_), and describe their appearance and habits.

2. =External characters.=—Kill a Dytiscus by dropping it for a moment into boiling water. Examine it first from the dorsal surface, noting the almost unbroken oval outline, and the firmness of the armour. Notice that the inner edges of the two wing-covers fit closely together in the middle line. How does the beetle compare in this respect with the cockroach? Observe the small triangular plate between the anterior ends of the wing-covers; open out the covers and wings to find of which segment of the thorax it is a part. Examine the wing-covers and wings closely; notice that the wings are folded transversely as well as longitudinally. Spread and pin out one wing-cover and wing of one side, and draw a dorsal view of the animal. Dissect out the jaws and compare them with the jaws of the cockroach (Fig. 222). Examine and draw one leg of each pair.

=The bordered little diver.=—In nearly every English pond there at times occurs a beetle—an inch or more in length—which is known to naturalists as _Dytiscus marginalis_. The name is somewhat cumbrous, but it would be difficult to find a more appropriate one; for “bordered little diver”—the plain English of the scientific term—indicates at once the peculiarities which soonest strike the observer: the creature’s skill in diving, and the yellow band which runs round the edge of the olive-green body. The outline of the beetle is an almost unbroken oval, and it is worth noticing that either such a boat-shape (Fig. 224, _B_), or the cigar-shape of the torpedo is adopted by almost all actively swimming aquatic animals. The resistance of the water is further lessened by the smoothness of the beetle’s armour, which forms a hard shell enclosing the body. As in all insects, the body is divided into head, thorax, and abdomen. The eyes are large, and are so arranged that the animal can at the same time see objects both above and below—a great advantage to a creature living so active a life. The feelers are very sensitive organs of touch, and possibly of smell also. The jaws consist of one pair of mandibles and two pairs of maxillae, and are shaped much like those of the cockroach; they are powerful, and render Dytiscus a very formidable enemy to the more peaceable inhabitants of the pond.

The thorax consists of the usual three segments, but only the first and a small triangular area of the second are to be seen until the wing-covers are pulled aside. These last differ from the wing-covers of the cockroach not only in being much harder and stronger, but also in their inner edges meeting accurately along the middle line of the body. The delicate, filmy wings, which alone are of use in flight, are folded transversely as well as longitudinally. On the lower side of the thorax are three pairs of legs; they are very interesting and are worth examining in some detail. In the male, the first leg on each side is furnished with little circular areas which were at one time believed to be suckers. It is now known, however, that they give off a sticky substance which adheres firmly to any object clasped by the legs. The middle pair of legs seems to be used chiefly for steering. In both sexes the third or last pair of legs is modified to form a pair of sculls. Ordinary land-beetles can move their legs in a vertical direction, as well as in a horizontal one; but the hind legs of water-beetles are jointed to the thorax in such a manner that they can only move backwards and forwards, not up and down. The resemblance of the hind leg to an oar does not stop here, however. On one side there is a fringe of stiff hairs, forming the blade of the oar. The joint carrying the hairs is so arranged that the beetle can “feather” its oar—by turning the edge of the blade to the water—at each stroke.

Occasionally, usually after sunset, the beetle quits his watery home, and “wheels his droning flight” in search of pastures new. His flights are, however, only temporary and merely from one pond to another. Although Dytiscus thus normally lives in water, very cursory observation only is needed to see that he cannot exist without a regular supply of fresh air. He no sooner stops paddling than his body rises naturally to the surface, and as the tail is lighter than the head, it rises out of the water. The wing-covers are now raised a little, so that the space between them and the wings is put into communication with the outside air. The impure gas contained in this space is soon replaced by a bubble of fresh pure air, the wing-cases are lowered, and the “little diver” plunges once more into the depths. The water is prevented by hairs from getting into the air-space below the wing-cases, and the true wings are thus kept always dry. The spiracles (p. 355) are in communication with the air-space, so that the animal is enabled to remain below the surface for a relatively long time.

=The life-history of Dytiscus.=—In March or April the female Dytiscus lays her eggs in slits which she cuts in the submerged stems of pond-weeds, and the eggs hatch in about three weeks. The creature which emerges from the egg is of active habits, but is not at all like the parent in appearance. A young animal which leads an independent and self-supporting life, and differs markedly in structure from the adult, is called a =larva=. Thus a tadpole is a larval frog, and a caterpillar is the larva of a butterfly or moth. The larva of Dytiscus when of full size is about 2 inches in length. Like other larvae of its family (Fig. 224, _A_), it has six slender legs, which serve both for swimming and for crawling over the bottom of the pond, and its head is provided with a pair of sickle-shaped mandibles, with which it seizes its prey. Each mandible is grooved on its inner side, the groove being converted into a tube by a membrane which covers it in. The savage larva sucks the blood of its victim until literally nothing is left but the shrivelled husk. At the end of the Dytiscus larva’s tail are two appendages which are fringed with hair. When the creature wishes to breathe it comes to the surface, and the tip of its tail protrudes out of the water. As each of the appendages just mentioned is pierced with a hole which leads into one of the two main air-tubes of the body, an interchange of vitiated for pure air readily takes place.

When the larva is about six weeks old, it leaves the pond and buries itself in the soil on the banks. Its exoskeleton is shed, and a thin, transparent layer of chitin—the “pupa-skin”—takes its place. In this condition the animal sinks into a state of torpor, and apparently becomes as motionless as a mummy. In this resting stage it is called a =pupa=. The pupal stage is necessary for the completion of the great changes—commenced some time previously—which must take place before the larva can acquire the structure of the adult. The pupal stage lasts two or three weeks, and when at last the creature emerges from its cell it is a beetle like its parents.

In consisting of three well-marked stages, the life-history of a beetle thus differs essentially from that of a cockroach. All beetles agree with Dytiscus in this respect, though in manner of life almost every conceivable variation is found. The beetle-order of insects receives its scientific name—=Coleoptera=[30]—from the sheathing character of the strong and closely-fitting wing-covers. The wings themselves are large, and folded in a somewhat complex manner. The mouth parts greatly resemble those of the cockroach.

64. BUTTERFLIES AND MOTHS.[31]

1. =Cabbage-white butterflies.=—(_a_) _The eggs._—In May or September, search the leaves of cabbages, turnips, and other crucifers (p. 95) for the tiny eggs of cabbage-white butterflies. Do the eggs occur singly or in clusters? Are they found on the upper or the lower surface of the leaf? Cut off a piece of leaf which carries eggs, put it under a tumbler, and examine it every day until the larvae (caterpillars) emerge from the eggs.

(_b_) _The larva._—Have ready a “breeding-cage,” _i.e._ a box measuring, say, 18” × 8” × 6”, one large face of which is of perforated zinc or fine wire gauze, and the other of glass; the box should be without bottom, so that it can be placed over a food-plant. Put the larvae, with leaves of the plant on which they were found, under the box, and observe them carefully. Replace all rotted or soiled leaves by fresh ones. It is best to keep the food-plant in a bottle of wet sand inside the case; the leaves then remain fresh for several days.

Describe the appearance of the caterpillar; notice its general worm-like form and absence of wings. In the _head_, observe the small feelers, and watch the action of the mandibles in feeding. Behind the head come the three segments of the _thorax_; notice that each bears a pair of short, jointed legs. How many segments can you see in the _abdomen_? Which of the abdominal segments bear legs or feet? How do these differ from the thoracic legs? Look for the spiracles (p. 355) at the sides of the body. Which segments have spiracles? Kill a full-fed caterpillar by immersing it in methylated spirit, or by putting it into a small box with a few drops of ether (_a most inflammable liquid_) or chloroform. When it is dead, examine it more closely. Try to make out the mouth-parts clearly; they are best seen from the back of the head. The labium (p. 353) is here represented by a conical body called the _spinneret_, out of which come the silken threads used to protect the pupa.

(_c_) _The pupa._—When the larva is full-fed, it seeks out a sheltered place, and fixes itself in position by means of silk threads which issue from the spinneret; the larval skin is shed and replaced by a pupal skin, and the animal remains quiescent until the change from larva to butterfly is complete. Examine the manner of attachment of the pupa to its support. Kill a pupa by immersion in methylated spirit, or by dipping it for a moment in boiling water; carefully strip off the skin, and make a drawing of the animal. Especially notice the arrangement of the wings and legs.

(_d_) _The imago or winged insect._—When the internal organs of the butterfly are completed, the pupal skin splits, and the perfect insect comes out. Look for pupae between September and March, or in July, and try to see the transformation. Notice the method of flight of the butterfly, and the position in which the wings are held when it settles. At what time of the day, and in what kind of weather, have you seen cabbage-whites flying? Kill a butterfly by putting it into a bottle containing crushed laurel leaves, or a few lumps of potassium cyanide (_a deadly poison_) wrapped in blotting paper, and examine it closely.

(i) _The head._—Notice the large, compound eyes, the knobbed feelers, the long and coiled _proboscis_, and the labial palps (which look like tusks).

(ii) The thorax.—The true form of the thorax and abdomen is concealed by the hairs which clothe the body. Wet the body with methylated spirit to make the hairs lie down. Notice that the first and third segments of the thorax are very small, while the middle segment, which carries the fore-wings, is large. Observe that the fore-wings are not modified into wing-covers, but are, generally speaking, much like the hind-wings. Determine the sex of the specimen by means of Fig. 225. Notice that, when you touch a wing, a little white dust comes off on your finger; the dust consists of very small scales. Do both surfaces of the wings bear scales? With a rather stiff camel-hair brush, brush the scales from the wings of one side. What markings have been removed, and what new markings have been made clear, by the removal of the scales? How many legs has the butterfly?

(iii) _The abdomen._—Count the segments.

2. =The tiger moth.=—In early summer examine lettuce, strawberry, and nettle leaves for “woolly bears”—the hairy caterpillars of the tiger moth. Keep these in the breeding-cage with the food-plants, and observe and describe their appearance and habits. How do they behave when alarmed? Watch them spin their cocoons (pupa cases) about the end of June, and describe the process. Examine the pupa, and state in what respects it differs from the caterpillar. About the end of July watch for the emergence of the perfect moth.

Notice the position of the wings of a resting moth. Are they held like those of a butterfly? Kill and examine the moth, and compare it further with a butterfly. Especially notice

(_a_) That the feelers of the tiger moth are not knobbed at the ends, but are either thread-like (in the female) or comb-like (in the male); and

(_b_) That a bristle at the base of the hind-wing hooks on a catch in the fore-wing of the same side.

3. =The vapourer moth.=—About the end of June examine rose trees, fruit trees, willows, oaks, etc., for caterpillars of the vapourer moth. They may be recognised (Fig. 228) by the reddish warts and the tufts of hair which stand out from various parts of the body. Notice that the caterpillars fall into two groups according to size, being, when full-grown, about 2 in. and 1¼ in. long respectively. Keep the larvae in a breeding-cage until they pupate and change into moths; notice that the moths (the females) which come from the large caterpillars have no wings; while the male moths, which are derived from the small caterpillars, have well-developed wings.

=A typical butterfly.=—The great beauty of butterflies and moths, and the ease with which the stages of their wonderful life-history can be followed, have made these insects favourite objects of study among naturalists of all ages.

Among the commonest of butterflies are the well-known =cabbage-whites=, the caterpillars of which work so much havoc upon crops of cruciferous plants (p. 95). The eggs are laid in May and September upon the leaves, and soon hatch out into small larvae called caterpillars, which feed voraciously and grow rapidly. The larval skin is shed from time to time as it becomes too small. The =caterpillar= (Fig. 225, _A_) is somewhat worm-like in appearance, but insect characters may easily be recognised in it. The _head_ is small and shiny. It carries six eye-spots, but the large compound eyes of the adult are not yet visible. A pair of short feelers is present; and the mouth-parts are obviously comparable with those of the cockroach, although the labium (p. 353) has become converted into a _spinneret_, which gives out the silk threads used for the protection of the pupa. The mandibles, which are used in gnawing leaves, are stout and toothed; the first maxillae are rather small. Behind the head come the three segments of the _thorax_, each of which bears a pair of short jointed legs; wings have not, as yet, developed. The _abdomen_ really consists of ten segments, although only nine can be seen without dissection. Segments 3 to 6 of the abdomen bear short, unjointed legs called _pro-legs_ or _cushion feet_, and the last segment bears a pair of appendages called the _anal feet_. Spiracles are to be seen on the first eight abdominal segments. In the larva of the green-veined cabbage-white, each spiracle is reddish and surrounded by a yellow border.

When the caterpillar has attained its full size, it stops feeding and seeks out a sheltered place—often a chink in a wall. Silken threads are given off by the spinnerets until a little heap of silk is formed into which the hooked end of the abdomen is fixed. Then a girdle of silk is made, passed round the thicker fore-part of the body, and so attached to the wall that the animal is supported in an upright position. The larval skin now splits and is peeled off, and the =pupa=, or chrysalis, stage (Fig. 225, _B_) is entered upon. All the external parts of the butterfly are complete at the time of pupation, but profound changes are still necessary in the internal organs; it is to allow these changes to take place in tranquillity that the resting, or pupal, stage is interposed between larval and adult life.

At last the new organs are ready for their work, the pupal skin cracks, and the =perfect insect= (Fig. 225, _C_) emerges. The head now carries a pair of large compound eyes, and two slender feelers with knobbed ends. The jaws also have been remodelled in accordance with the completely different manner of life upon which the insect is now entering. The mandibles are now only doubtfully recognisable; the first pair of maxillae are elongated and grooved, and are closely applied to each other to form a long tube called the _proboscis_ (Fig. 226, _Mx._ 1), which, when not in use for sucking up the sweet juices of flowers, is kept coiled up beneath the head; the palps of the labium (_Lab. Pa._) project like tusks on the sides of the head. The thorax is provided with two pairs of broad wings, which are covered with minute overlapping scales—forming a delicate “bloom” which is readily detached by rough handling. In many butterflies and moths the scales are gorgeously coloured and arranged in symmetrical patterns. The name =Lepidoptera=,[32] which is applied to this order of insects, was suggested by the scaly covering of the wings. When the butterfly is at rest the wings are either fully expanded horizontally, or are held vertically over the back, the upper surfaces of the fore wings being in contact. The first segment of the thorax, which bears the first of the three pairs of legs, is greatly reduced; the third segment also is but small.

In short, the whole organisation of a butterfly is definitely adapted to the special duties which belong to this period of its life. The growing stage is over; the sole object of life is now to seek a mate and—in the case of the female—to lay the eggs in a place where the future larvae may find plenty of food. Ease of locomotion and conspicuousness are secured by the broad and brilliantly-coloured wings; the peculiar manner of flight is considerable protection against the attacks of birds; large eyes aid in the recognition of the mate; and a concentrated and easily-digested food is supplied by the nectar of flowers, and made accessible by the long, sucking proboscis—the service of cross-pollination (p. 92) often being unconsciously rendered in return for the sweet draught.

The common species of cabbage-white butterflies spend the winter as pupae; the perfect insects emerge in April, lay their eggs, and then die. The caterpillars pupate and a second generation of butterflies appears, their offspring reaching the pupa stage about the end of autumn.

=Moths.=—Moths pass through a life-history which is identical, in its broad features, with that of butterflies. The larva which hatches from the egg is a caterpillar, whose life is spent in feeding and growing. At the same time the external features of the adult are gradually taking form under the skin. When at last the full larval size is attained, and a resting stage is necessary for the perfection of the internal organs, the caterpillar’s skin splits, and is shed, and the animal becomes a pupa or chrysalis. A moth-pupa is generally somewhat egg-shaped (Fig. 227), whereas the pupa of a butterfly is usually conical, though there are many exceptions.

The winged moth, which at length emerges from the pupal skin, differs from a butterfly in certain obvious respects. Its body is usually broad and thick; its feelers are either comb-like or thread like, not knobbed at the ends; the two wings of one side are in most cases secured together at the base by one or more bristles on the hind-wing hooking over a catch on the fore-wing; in rest, the wings usually slope and are not fully extended. Whereas butterflies usually fly only in the sunshine, moths often fly by night, and the flowers which night-flying moths frequent for nectar are as a rule white and strongly-scented, and close during the day. In finding their mates, moths seem to depend largely upon the sense of smell, which is probably lodged in the feelers.

The life-history of a typical moth is well exemplified by the =Tiger Moth= (Fig. 227), which is easily reared in captivity. The larva—often called the “woolly bear,” from its thick covering of hair—may be found in early summer on the leaves of lettuce, strawberry, nettle, and other plants. It pupates about the end of June, working its hair, together with silk spun by the spinneret, into a _cocoon_, in which the resting stage is passed. About a month later the perfect moth emerges; its fore-wings are beautifully mottled with cream colour and chocolate brown; the hind-wings are red, with metallic violet spots. The feelers of the male are comb-like, and are probably very sensitive organs of smell, by means of which he seeks out his mate. The female’s feelers are thread-like.

In the =Vapourer Moth= (Fig. 228), whose “looping” flight may often be observed even in the streets of towns during the day, the two sexes are remarkably different from each other. The male (_C_) alone can fly; the female (_D_) is wingless, and is confined for the whole of her short adult life to the place where she emerged from the cocoon. Here she lays her eggs and then dies. Neither she nor her mate is capable of feeding.

It would be difficult to find a more striking example of the fact that the one duty of the adult moth is reproduction. The female vapourer is even debarred from the privilege of choosing favourable places for her eggs; but a compensation for this disadvantage lies in the agility of the larvae (_A_), which are able to migrate without difficulty to another plant whenever food becomes scarce. As Prof. Miall remarks:[33] “Whatever the larva can do for itself, the female can safely leave undone; but what the larva cannot do, by reason of sluggishness or restricted diet, the parent must provide for. Hence activity and intelligence in the one lead to degeneration in the other.... Wings are to insects what spores are to ferns, plumed seeds to dandelions, and hooked seeds to burrs—a ready means of dispersal.”

=Other insects.=—Space does not permit of more than a reference to other insects, and the work of this chapter is to be regarded as the merest introduction to the study of this fascinating class of animals. The chief remaining orders are the =Neuroptera=, including May flies, dragon flies, and caddis flies; the =Hemiptera=, among which are included the various “bugs,” water boatmen, plant lice, etc.; the =Diptera= (two-winged flies), such as the house-fly, gnat, harlequin fly, daddy-long-legs, etc.; and the =Hymenoptera=, including bees, wasps, ants, gall flies (p. 146), and ichneumons. Many of these insects have aquatic larvae which may be found in ponds; and their life-histories should be studied in aquaria and careful notes made of the transformations.[34]

EXERCISES ON CHAPTER XIX.

1. Examine the following animals, and find out (_a_) which are arthropods, (_b_) which are insects: tortoise, spider, grasshopper, lobster, earwig, centipede. Give reasons for your conclusions.

2. Keep a grasshopper under a tumbler with a small sod of grass. Observe its habits, and find out how it “chirps.” Compare its structure with that of the cockroach.

3. Compare a cockchafer with a water-beetle. In what order of insects would you place the cockchafer, and why?

4. Compare other water-beetles with Dytiscus, and try to trace their life-history.

5. Observe the habits and examine the structure of the water-boatman. What reasons can you find for excluding it from the beetle-order?

6. Examine a daddy-long-legs, and try to find the two stumps which are all that remain of the hind-wings.

7. Look for “blood worms” (larvae of the harlequin fly) in horse-troughs and sluggish streams in summer; keep them in a saucer of water with a few dead leaves. Observe their habits and describe the appearance of the pupa. What kind of insect emerges from the pupa?

8. Keep caddis-worms in an aquarium and describe their habits.

9. Examine the leaves of stinging nettles for caterpillars in June, and try to rear butterflies from them. Carefully notice from which kind of caterpillar each butterfly is derived.

10. Look for caterpillars of the Privet Hawk Moth on privets and lilacs on August and September evenings. Keep some, with earth and twigs of the food plant, in a covered flower-pot, and observe their method of pupation.

11. Compare the colouration of the wings of butterflies and moths with that of the plants they most frequent, and describe any cases of protective colouration which you find.

FOOTNOTES:

[28] Greek: _orthos_, straight; _pteron_, a wing.

[29] See Footnote, p. 372.

[30] Greek: _koleos_, a sheath; _pteron_, a wing.

[31] Living eggs, larvae, and pupae of Lepidoptera may be obtained from Mr. H. W. Head, Burniston, near Scarborough, and other dealers.

[32] Greek: _lepis_, a scale; _pteron_, a wing.

[33] _Injurious and Useful Insects_ (Bell).

[34] Miall’s _Injurious and Useful Insects_ (Bell), and _Natural History of Aquatic Insects_ (Macmillan), are strongly recommended as guides to further work.