Part 7

Usually swimming feet. Legs tolerably long, adapted for wading (Fig. 45). Wings long, pointed. Beak laterally compressed. Young nestlings (p. 51). Breed in larger or smaller flocks, usually on the coast, occasionally on the margin of fresh waters. They chiefly feed on fish, worms, molluscs, and crustacea; sometimes, in the case of a few species, on young birds and eggs, as well as mice and other small mammals. Gulls are usually of no importance in agriculture; but the =Black-headed Gull= (_Larus ridibundus_), which breeds on the banks of lakes and rivers, devours many cockchafers and other noxious insects. The =Herring Gull= (Fig. 45), =Kittiwake= (_Larus argentatus_ and _L. tridactylus_), and a few other species, which breed on the coast, sometimes show themselves inland, especially in stormy weather; they then pursue field-voles, and catch many injurious insects.

Besides the true =Gulls= (_Larus_), I will only mention the =Sea Swallows= (_Sterna_), with forked tails.

CLASS III.: =REPTILIA= (REPTILES).

Cold-blooded Vertebrates (p. 19). The heart has the same structure as in Mammals and Birds, but the left and right ventricles are usually only incompletely divided from one another (p. 20). The body is invested with horny scales or with bony plates covered by a horny layer. Reptiles are very unlike Birds in appearance, but present many essential points of resemblance to them in their skeleton; indeed, during earlier geological times, there were transitional forms between the two classes. The structure of the reptile egg, too, is very similar to that of the bird’s egg; but the first has no lime-salts deposited in its shell. The egg is not hatched out by the animal itself, but exposed to the heat of the sun or to the warmth developed by decaying vegetable matter. Several reptiles (adder, for example) keep their eggs in their bodies till the young escape. Reptiles have either no limbs (snakes, a few lizards), or, at any rate, the limbs are not well developed, and are so placed that the body does not rest upon them, but is slung between them.

The Reptilia are divided into the Orders of =Crocodilia= (Crocodiles), =Chelonia= (Turtles and Tortoises), =Lacertilia= (Lizards), =Ophidia= (Snakes).

Our native reptiles have no agricultural importance. I will, however, briefly mention the =Adder= (_Pelias berus_, Fig. 47), as it is dangerous to man. Head broad; tail much smaller than the hinder end of the body. Colour greenish grey or brownish. A black zigzag band runs along the dorsal side of the body. Length about twenty inches. The two hook-shaped poison-fangs are found in the front of the upper jaw; opening the mouth widely causes them to be erected. They are traversed by poison-canals through which the poison flows immediately into the two bloodless wounds. The adder lives in woods and on mountain slopes, where it devours mice. The poison has a decomposing action on the blood; it causes fever, and swelling of the part bitten, as well as sometimes of the neighbouring parts. The bite may be fatal. Alcohol is spoken of very highly as a remedy.

The native =Ringed Snake= (_Tropidonotus natrix_, Fig. 48), three feet long, is bluish with belly flecked with black and white; a yellow and a black band on the neck. It is fond of going into the water, and eats frogs, newts, and insects. The =Smooth Snake= (_Coronella lævis_), two feet long, is brown, with black patches on the back, which, however, do not form a continuous band. Head dark velvety black. Devours lizards and blindworms. The =Blindworm= (_Anguis fragilis_) is indeed snake-like, or rather worm-like (Fig. 49), but nevertheless belongs to the lizards, with which its internal structure agrees.

CLASS IV.: =AMPHIBIA= (AMPHIBIANS).

Cold-blooded Vertebrates (p. 19). The heart has only one ventricle and two auricles (cp. p. 21). The skin is naked, damp, usually slippery and smooth; it helps in respiration. Although several Amphibians resemble various Reptiles in outward appearance (Newt and Lizard, Figs. 50 and 46), the structure of the skeleton is quite different. Upon the whole, Amphibians resemble fishes more closely, and they breathe by gills when they are young, which strengthens the agreement. They undergo a metamorphosis. The just-hatched young at first hold fast by suction to the jelly of the spawn; they have external gills. They quickly develop a membranous margin to the body, especially round the tail, which consequently forms a sort of oar. Meanwhile internal gills develop, and the external ones disappear. The larvæ are now fish-like (“tadpoles”). The limbs quickly grow out, the lungs develop, the tail shrivels up, and the animal leaves the water. Even the adult, however, lives among damp surroundings. In the adult state the Amphibians feed on insects, worms, and snails; many species are exceedingly useful owing to this.

Amphibians are divided into two orders, the _Urodela_ (Newts, etc.) and the _Anura_ (Frogs and Toads). To the first belong the =Great Crested Newt= (_Triton cristatus_) and the =Common Newt= (_Lissotriton tæniatus_). Examples of the Anura are the =Edible Frog= (_Rana esculenta_), the brownish =Common Frog= (_R. temporaria_, Fig. 51), the =Common Toad= (_Bufo vulgaris_), and the =Natterjack= (_B. calamita_, Fig. 52).

Both frogs and toads do good by devouring many noxious insects, and also, in particular, snails. Toads are often kept in greenhouses for this purpose; and in the research garden attached to the Rouen entomological laboratory the snails were entirely exterminated in 1891, as a result of introducing a hundred toads and ninety frogs.

CLASS V.: =PISCES= (FISHES).

Cold-blooded Vertebrates (p. 19), which breathe by gills during their whole life. The heart consists only of a single ventricle and a single auricle (p. 18). The head passes immediately into the body without any intervening neck. Fishes move chiefly by means of the tail, at the end of which there is a tail-fin. This and the dorsal and anal fins lie in the median plane of the fish, while the pectoral fins, which are attached to the skull in bony fishes, and also the ventral fins, are paired structures more or less comparable to the fore and hind limbs of higher vertebrates. The skeleton of most fishes (pike, perch, carp, eel, plaice) is bony; but in a few subdivisions of fishes (sharks, skates, sturgeons, lampreys) it is cartilaginous. The skin is usually covered with thin translucent scales; but there are fishes with a smooth skin (lampreys), with prickle-like dermal ossifications (sharks), and with rhomboidal bony plates (sturgeons). As Fishes are without exception aquatic, there is no need for me to treat more specially of them here.

Second Sub-Kingdom: ARTHROPODA (JOINTED-LIMBED ANIMALS).

The body of an Arthropod is bilaterally symmetrical (cp. p. 16), and consists of a number of joints (Fig. 55), not equally numerous in all members of the sub-kingdom. These joints or segments lie one behind the other, and are at first alike; but as in the course of further development they become adapted to various functions, the difference between them becomes greater. Compare the Wood-borer represented in Fig. 54, 2, with the young form of the same animal (Fig. 54, 1). The segments often fuse together, which brings about the formation of a smaller number of subdivisions to the body; or even all the segments may become united (mites). In the last case the Arthropod characters are only to be seen in the jointing of the limbs. Segmented animals (_e.g._ the common earthworm) are also found among the worms (Sub-kingdom III. of the Animal Kingdom); but these worms have either no limbs, or only small unjointed foot-stumps, never jointed limbs as in the Arthropods. These last are limbless or with unjointed foot-stumps only when young; when adult they always have jointed limbs. The head of Arthropods bears several successive pairs of jaws, which move to and fro from one side towards the other. The covering of the body consists of hard protective pieces; it is only in the young condition that the skin of several species is soft. Arthropods have no internal skeleton; the muscles are attached to the skin. The central part of the nervous system (p. 10), which consists in Vertebrates of brain and spinal cord, lies in Arthropods almost entirely on the ventral side. In the head is situated the cerebral ganglion, a large nervous mass resting on the gullet, and giving off nerves to the eyes and feelers. Besides this, there is a ventral nervecord on the ventral side of the animal, running below the gut, and made up of several pairs of ganglia, united with one another by means of nerve fibres. The ganglia of the ventral cord send their nerves to the jaws, limbs, muscles, viscera, etc. The cerebral ganglion is connected with the first ventral ganglion by means of a cord on each side of the gullet (Fig. 56), so that a ring-like structure is formed. The breathing organs are gills in some Arthropods (the Crustacea, _e.g._ crayfish, crabs); insects and centipedes breathe by air-tubes or tracheæ, while the respiratory organs of spider-like animals, when these do not breathe entirely by means of the skin, are more or less strongly modified tracheæ. The structure of the tracheal system is generally as follows: on each side of the body there is a row of breathing-holes, or stigmata, through which the air can enter the tracheæ; these are very much branched, so that they finally become very fine tubes investing the various organs, which in this way obtain the requisite amount of oxygen. The stigmata on one side of a caterpillar are clearly shown in Fig. 60, and those of a hornet larva in Fig. 61.

Four classes belong to this sub-kingdom: Insecta (Insects), Myriapoda (centipedes, etc.), Arachnoidea (spiders, scorpions, etc.), Crustacea (crayfish, crabs, lobsters, etc.).

CLASS I.: =INSECTA= (INSECTS).

Breathe by tracheæ (cp. p. 84). The segments fuse into three body regions (Fig. 57). These are—(1) the _Head_, bearing the eyes, feelers (antennæ), and jaws; (2) the _Thorax_, composed of three fused segments, of which the first (prothorax) bears a pair of legs, while the second (mesothorax) and third (metathorax) not only bear a pair of legs each, but also sometimes a pair of wings in addition; (3) the _Abdomen_, which possesses no limbs, and of which the number of segments is not always the same. These three regions of the body perform different functions; the head is concerned with sensation and the taking up of food, the thorax with movement, while the abdomen contains the organs of digestion and reproduction. I will now deal somewhat more fully with the different body regions of an insect.

_Head._—Almost all insects have in the adult state a compound eye, _i.e._ an eye made up of a large number (up to ten thousand) of smaller eyes. In many insects one finds in addition to this a few simple eyes on the top of the head. The feelers are very unlike in different insects; they serve as organs of touch, and perhaps also help some other sense. The mouth-parts consist of three pairs of jaws, of which the first (the mandibles) and the second pair (maxillæ) are freely movable from side to side, while the third consists of two jaws immovably fused together (lower lip, or labium). A downward projection of the skin of the head (the upper lip, or labrum) overhangs the three pairs of jaws (Fig. 58). In insects which feed on solid matters, tearing or chewing them, the jaws are short and sharp-edged (biting mouth-parts); in those which take up fluid food (blood, the juices of plants, etc.) they are elongated, and adapted for licking, sucking, or piercing.

_Thorax._—The names of the parts of the legs are for the most part those used in the case of mammalian limbs; though the similarity between the limbs of Mammals and Insects is quite superficial. The following parts are distinguished in the leg of an insect: (1) the generally spherical hip (_coxa_) (Fig. 59, _a_); (2) the very short _trochanter_ (_b_); (3) the elongated thigh (_femur_) (_c_); (4) the shank (_tibia_) provided with movable spines at its tip (_d_); (5) the three- to five-jointed foot (_tarsus_) (_e_), whose last joint ends in claws, and often also in flap-like outgrowths. The _Wings_ are expansions of the skin, which consist of two layers. There are tracheæ between the upper and lower lamellæ. At first (in the pupa) the wings are folded up; but the forcing of air into the tracheæ quickly causes them to unfold. A firm substance is now deposited in the larger tracheæ, which are thus converted into veins or ribs giving support to the wings. In beetles the fore wings are quite hard and horny, and serve for protecting the delicate hind wings and soft back rather than for flight; they are therefore named “wing-covers” (_elytra_). In many insects the fore wings (wasps) or the hind wings (beetles, grasshoppers) are folded when at rest.

The _abdomen_ only bears organs of movement in caterpillars (Fig. 60) and a few other insects in the young condition, and in these cases they are not jointed like the true legs or “thoracic feet,” but unjointed pro-legs or “abdominal feet.” In the adult insect the abdomen may have thread-like (mole-cricket) or pincer-like (earwig) appendages; or appendages used in egg-laying (ovipositors, _e.g._ Locust).

Most insects have great powers of reproduction. A few bring forth living young, but most insects lay eggs. It is only in a few (_e.g._ lice) that animals exactly like their parents are hatched from the eggs; the large majority of insects pass through a _change_, or _metamorphosis_.

A distinction is drawn between _complete_ and _incomplete_ metamorphosis. It is said to be complete when the insect passes through a stage in which it takes up no food, and, as a rule, moves but little. In this condition of almost complete rest the insect is called a “pupa” (Figs. 60, 61, 65). The metamorphosis is called incomplete when the insect does not pass through a pupa stage, and therefore feeds and moves during all the periods of its development, merely altering its form somewhat during its various moults (Fig. 62).

The word “moult” must here be explained. The covering of the skin of Arthropods consists of hard parts incapable of being stretched. At the end of a definite period of time this firm covering is stripped off and replaced by a new investment, soft at first, but afterwards becoming hard. In this way growth can be effected in spite of the hard integument. Everyone must have noticed at some time or other the moulting of caterpillars.

In insects with incomplete metamorphosis the form of the animal alters a little at every moult, becoming more and more like that of the adult insect. In the last moult but one small imperfect wings appear (Fig. 62, _left_), and fully developed wings are only present after the last moult. The ovipositors of female insects, in those cases where the metamorphosis is incomplete, first appear as fully developed organs in the perfect condition of the animal, but begin to develop in the preceding stage. In cases of this kind of metamorphosis the young insect (“larva”) closely resembles the adult in form even in the first stage of development. In insects undergoing complete metamorphosis the difference between larva and adult insect (imago) is much greater (Figs. 60, 61, 65).

The time passed in the pupa state by insects with complete metamorphosis is by no means always of the same length. For example, there are two generations annually of the cabbage white butterfly; one lives through the winter in the pupa state, the pupæ of the other are found in summer. So that while an insect of the winter generation lives about half a year in the pupa state, this condition lasts only about a month in the summer generation. A higher temperature hastens the development.

Although the insect in the pupa state takes no food, it nevertheless breathes, and therefore continually uses up body substance. This using up only takes place to a small extent however, since the animal moves but little. From whence, then, does the pupa get the material to cover the loss of body substance? In the larval condition the insect takes far more food than it requires for the development of its body, and from this excess it builds up reserve stuffs, which are deposited in the so-called “fat-bodies” of the larva. These reserve materials are re-absorbed during the pupa state, and serve to maintain the breathing. Consequently a pupa that has just been formed moves more than another one just about to become a butterfly.

The larvæ and pupæ of the different kinds of insects which undergo a complete metamorphosis have not the same shape of body. Among the larvæ may be distinguished _caterpillars_, _grubs_, and _maggots_. The _caterpillars_ (Fig. 60) have a clearly marked head with hard covering, three pairs of jointed thoracic legs, and a varying number of unjointed pro-legs. They are usually variegated or green in colour, and are divided into _true caterpillars_ and _false caterpillars_ (Figs. 60, 63, and 64). The first, after the resting pupa stage, become butterflies or moths; the latter saw-flies. The true caterpillars have two to five pairs of pro-legs, the false caterpillars six to eight pairs. Reckoning in the thoracic legs, therefore, the true caterpillars have altogether five to eight, the false caterpillars nine to eleven pairs. The head of the latter is more rounded, while that of the former is more flattened. The way in which caterpillars walk depends upon the number of their legs. If this is fairly large so that most of the segments of the body are provided with legs, the whole body remains tolerably extended during progression. But if the number is small—as in the looper caterpillars, where there are three pairs of thoracic legs at the anterior end, and at the posterior end only a terminal pair of legs (the caudal pro-legs) and another pair in front of them—the middle legless region is strongly bent during locomotion (Fig. 63). Hence the name “looper.” The loopers often bend their bodies in a characteristic way. When at rest the hinder part is coiled up spirally; but as soon as the animals are alarmed they throw the hinder part of the body upwards and forwards and even over the head. The _grubs_ (Figs. 66 and 69, _left_) have indeed, like the caterpillars, a clearly visible hard head, but no characteristic abdominal legs, or at most a pair of sucker feet at the end of the body (wireworms). Thoracic legs are present in several grubs (cockchafer larvæ, wireworms, leaf beetles); in others (larvæ of weevils and fleas) they are entirely absent. _Maggots_ are those entirely footless insect larvæ which do not possess a head clearly marked off from the rest of the body, and the head-end of which is only to be recognized by the presence of the mouth and mouth-parts. _Pupæ_ are enclosed in a case which either only faintly indicates the outline of the various parts of the adult insect, or else closely surrounds every part of the body—wings, legs, antennæ, and even the mouth-parts and eyes. Pupæ of the first kind are termed _obtectate_ (Fig. 60); those of the second, _free_ (Figs. 61, 65).

Many pupæ are _naked_, others are surrounded by a web (cocoon) spun by the larva (Fig. 60). There are also pupæ distinguished by the peculiarity that when the insect has lived through the maggot-stage it does not strip off its integument, but turns into a pupa inside the shrivelled maggot-skin, from which the perfect insect later on breaks out (Fig. 67).

The class of insects can be divided into eleven orders: (1) _Coleoptera_ (Beetles); (2) _Orthoptera_ (Grasshoppers, Locusts); (3) _Neuroptera_ (Dragon-flies); (4) _Hymenoptera_ (Bees, Ants, Saw-flies); (5) _Lepidoptera_ (Butterflies and Moths); (6) _Hemiptera_ (Aphides, Bugs); (7) _Physopoda_ (Thrips); (8) _Diptera_ (Flies with two wings); (9) _Aphaniptera_ (Fleas); (10) _Pediculina_ (Lice); (11) _Collembola_ (Spring-tails and Tassel-tails).

ORDER I.: =Coleoptera= (BEETLES).

Beetles (Fig. 65) are insects with biting mouth-parts, and strongly developed prothorax united with the mesothorax so as to permit free movement. The fore wings are in the form of hard covers, leaving exposed only the head, neck-shield (_i.e._ the dorsal side of the prothorax), a three-cornered bit of the mesothorax (_scutellum_), and sometimes the tip of the abdomen. Flight is effected by the hind wings alone, which in a state of rest are drawn back under the wing-covers. The metamorphosis is complete; the larvæ are legless, or with thoracic legs only, and have a hard head with biting mouth-parts; change into free pupæ (p. 93).

Family: =Carabidæ= (_Ground Beetles_).

Usually elongated, slender; with long slender legs, five-jointed tarsi, eleven-jointed antennæ, powerful jaws (Figs. 68, 69). Run rapidly; usually keep on the ground; hide themselves during the day, but are very active at night; with very few exceptions feed entirely on other insects; when touched squirt an acrid stinking fluid out of the abdomen. Larvæ longish, six-legged, with short antennæ and sharp jaws, with a few exceptions live exclusively on other insects and lower animals.

Several species are of service, both in the adult and larval conditions, since they destroy injurious insects, _e.g._ surface caterpillars, wireworms, cockchafers, grubs, crane-fly larvæ. The following do good in cultivated fields: =Golden Ground Beetle= (_Carabus auratus_), =Garden Ground Beetle= (_C. hortensis_), =Granulated Ground Beetle= (_C. granulatus_), =Cross-barred Ground Beetle= (_C. cancellatus_), =Field Ground Beetle= (_C. nemoralis_), =Large-headed Ground Beetle= (_Cephalotes vulgaris_), species of _Harpalus_, _Pterostichus_, etc.