Anatomy and General Characteristics of the Class Insecta
The body of an insect is divided by means of two marked narrowings into three parts: the head, the chest, and the abdomen.
Fig. 122.—Yellow Fever Mosquito, showing head, thorax, abdomen.
The head is a freely movable capsule bearing four pairs of appendages. Hence it is regarded as having been formed by the union of four rings, since the ancestor of the insects is believed to have consisted of similar rings, each ring bearing a pair of unspecialized legs.
The typical mouth parts of an insect (Fig. [123]) named in order from above, are (1) an upper lip (labrum, ol), (2) a pair of biting jaws (mandibles, ok), (3) a pair of grasping jaws (maxillæ, A, B), and (4) a lower lip (labium, m, a, b). The grasping jaws bear two pairs of jointed jaw fingers (maxillary palpi, D, C), and the lower lip bears a pair of similar lip fingers (labial palpi, d). The biting jaws move sideways; they usually have several pointed notches which serve as teeth. Why should the grasping jaws be beneath the chewing jaws? Why is it better for the lower lip to have fingers than the upper lip? Why are the fingers (or palpi) jointed? (Watch a grasshopper or beetle eating.) Why does an insect need grasping jaws?
Fig. 123.—Mouth Parts of Beetle.
Fig. 124.—External Parts of a Beetle.
The chest, or thorax, consists of three rings (Fig. [124]) called the front thorax (prothorax), middle thorax (mesothorax) and hind thorax (metathorax), or first, second, and third rings. The first ring bears the first pair of legs, the second ring bears the second pair of legs and the upper or front wings, and the third ring bears the third pair of legs and the under or hind wings.
Fig. 125.—Leg of Insect.
The six feet of insects are characteristic of them, since no other adult animals have that number, the spider having eight, the crayfish and crabs having ten, the centipedes still more, while birds and beasts have less than six. Hence the insects are sometimes called the Six-Footed class (Hexapoda). The insects are the only animals that have the body in three divisions. Man, beasts, and birds have only two divisions (head and trunk). Worms are not divided.
Define the class insecta by the two facts characteristic of them (i.e. possessed by them alone), viz.: Insects are animals with ———— and ————. Why would it be ambiguous to include “hard outer skeleton” in this definition? To include “bilateral symmetry”? “Segmented body”? The definition of a class must include all the individuals of the class, and exclude all the animals that do not belong to the class.
Fig. 126.—Foot of Fly, with climbing pads.
The leg of an insect (Fig. [125]) has five joints (two short joints, two long, and the foot). Named in order from above, they are (1) the hip (coxa), (2) thigh ring (trochanter), (3) thigh (femur), (4) the shin (tibia), (5) the foot, which has five parts. Which of the five joints of a wasp’s leg (Fig. [161]) is thickest? Slenderest? Shortest? One joint (which?) of the foot (Fig. [161]) is about as long as the other four joints of the foot combined. Is the relative length of the joints of the leg the same in grasshoppers, beetles, etc., as in the wasp (Figs.)? Figure 125 is a diagram of an insect’s leg cut lengthwise. The leg consists of thick-walled tubes (o, n) with their ends held together by thin, easy-wrinkling membranes which serve as joints. Thus motion is provided for at the expense of strength. When handling live insects they should never be held by the legs, as the legs come off very easily. Does the joint motion of insects most resemble the motion of hinge joints or ball-and-socket joints? Answer by tests of living insects. There are no muscles in the foot of an insect. The claw is moved by a muscle (m) in the thigh with which it is connected by the long tendon (z, s, t, v). In which part are the breathing muscles? As the wings are developed from folds of the dorsal skin, the wing has two layers, an upper and a lower layer. These inclose the so-called “nerves” or ribs of the wing, each of which consists of a blood tube inclosed in an air tube.
The abdomen in various species consists of from five to eleven overlapping rings with their fold-like joints between them. Does each ring overlap the ring in front or the one behind it?
Fig. 127.—Viscera of Grasshopper. Key in text. Compare with Fig. [114].
Fig. 128.—Air Tubes of Insect.
The food tube (Fig. [127]) begins at the mouth, which usually bears salivary glands (4, Fig. [127], which represents internal organs of the grasshopper). The food tube expands first into a croplike enlargement; next to this is an organ (6, Fig. [127]), which resembles the gizzard in birds, as its inner wall is furnished with chitinous teeth (b, Fig. [114]). These reduce the food fragments that were imperfectly broken up by the biting jaws before swallowing. Glands comparable to the liver of higher animals open into the food tube where the stomach joins the small intestine. At the junction of the small and the large intestine (9) are a number of fine tubes (8) which correspond to kidneys and empty their secretion into the large intestine.
The breathing organs of the insects are peculiar to them (see Fig. [128]). They consist of tubes which are kept open by having in their walls continuous spirals of horny material called chitin. Most noticeable are the two large membranous tubes filled with air and situated on each side of the body. Do these tubes extend through the thorax? (Fig. [128].) The air reaches these two main tubes by a number of pairs of short windpipes, or tracheas, which begin at openings (spiracles). In which division are the spiracles most numerous? (Fig. [128].) Which division is without spiracles? Could an insect be drowned, i.e. smothered, by holding its body under water? Could it be drowned by immersing all of it but its head? The motion of the air through the breathing tubes is caused by a bellowslike motion of the abdomen. This is readily observed in grasshoppers, beetles, and wasps. As each ring slips into the ring in front of it, the abdomen is shortened, and the impure air, laden with carbon dioxide, is forced out. As the rings slip out, the abdomen is extended and the fresh air comes in, bringing oxygen.
Fig. 129.—Insect’s Heart (plan).
Fig. 130.—Diagrams of Evolution of Pericardial Sac around insect’s heart from a number of veins (Lankester).
Fig. 131.—Position of Insect’s Heart, food tube, and nerve chain.
The Circulation.—Near the dorsal surface of the abdomen (Fig. [131]) extends the long, slender heart (Fig. [129]). The heart has divisions separated by valvelike partitions. The blood comes into each of the heart compartments through a pair of openings. The heart contracts from the rear toward the front, driving the blood forward. The blood contains bodies corresponding to the white corpuscles of human blood, but lacks the red corpuscles and the red colour. The blood is sent even to the wings. The veins in the wings consist of horny tubes inclosing air tubes surrounded by blood spaces, and the purification of the blood takes place throughout the course of the circulation. Hence the imperfect circulation is no disadvantage. The perfect provision for supplying oxygen explains the remarkable activity of which insects are capable and their great strength, which, considering their size, is unequalled by any other animals.
Fig. 132.—Nervous System of Bee.
Fig. 133.—Feeler of a beetle.
The Nervous System.—The heart in backboned animals, e.g. man, is ventral and the chief nerve trunk is dorsal. As already stated, the heart of an insect is dorsal; its chief nerve chain, consisting of a double row of ganglia, is near the ventral surface (Fig. [131]). All the ganglia are below the food tube except the first pair in the head, which are above the gullet. This pair may be said to correspond somewhat to the brain of backboned animals; the nerves from the eyes and the feelers lead to it. With social insects, as bees and ants, it is large and complex (Fig. [132]). In a typical insect they are the largest ganglia.
The Senses.—The sense of smell of most insects is believed to be located in the feelers. The organ of hearing is variously located in different insects. Where is it in the grasshopper? The organs of sight are highly developed, and consist of two compound eyes on the side of the head and three simple eyes on the top or front of the head between the compound eyes. The simple eye has nerve cells, pigments, and a lens resembling the lens in the eyes of vertebrates (Fig. [134]). The compound eye (Fig. [135]) has thousands of facets, usually hexagonal, on its surface, the facets being the outer ends of cones which have their inner ends directed toward the centre of the eye. It is probable that the large, or compound, eyes of insects only serve to distinguish bright objects from dark objects. The simple eyes afford distinct images of objects within a few inches of the eye. In general, the sight of insects, contrary to what its complex sight organs would lead us to expect, is not at all keen. Yet an insect can fly through a forest without striking a twig or branch. Is it better for the eyes that are immovable in the head to be large or small? Which has comparatively larger eyes, an insect or a beast?
Fig. 134.—Diagram of simple eye of insect.
L, lens; N, optic nerve.
Fig. 135.—Compound Eye of Insect.
1, hexagonal facets of crystalline cones. 6, blood vessel in optic nerve.
Inherited Habit, or Instinct.—Insects and other animals inherit from their parents their particular form of body and of organs which perform the different functions. For example, they inherit a nervous system with a structure similar to that of their parents, and hence with a tendency to repeat similar impulses and acts. Repeated acts constitute a habit, and an inherited habit is called an instinct. Moths, for example, are used to finding nectar in the night-blooming flowers, most of which are white. The habit of going to white flowers is transmitted in the structure of the nervous system; so we say that moths have an instinct to go to white objects; it is sometimes more obscurely expressed by saying they are attracted or drawn thereby.
Instincts are not Infallible.—They are trustworthy in only one narrow set of conditions. Now that man makes many fires and lights at night, the instinct just mentioned often causes the death of the moth. The instinct to provide for offspring is necessary to the perpetuation of all but the simplest animals. The dirt dauber, or mud wasp, because of inherited habit, or instinct, makes the cell of the right size, lays the egg, and provides food for offspring that the mother will never see. It seals stung and semiparalyzed spiders in the cell with the egg. If you try the experiment of removing the food before the cell is closed, the insect will bring more spiders; if they are removed again, a third supply will be brought; but if taken out the third time, the mud wasp will usually close the cell without food, and when the egg hatches the grub will starve.
The Development of Insects.—The growth and the moulting of the grasshopper from egg to adult has been studied. All insects do not develop exactly by this plan. Some hatch from the egg in a condition markedly different from the adult. The butterfly’s egg produces a wormlike caterpillar which has no resemblance to the butterfly. After it grows it forms an inclosing case in which it spends a quiet period of development and comes out a butterfly. This change from caterpillar to butterfly is called the metamorphosis. The life of an insect is divided into four stages: (1) egg, (2) larva, (3) pupa, and (4) imago, or perfect insect (Figs. [136], [137], [138]).
Fig. 136.—Measuring worm, the larva of a moth.
Fig. 137.—Pupa of a mosquito.
The egg stage is one of development, no nourishment being absorbed. The larval stage is one of voracious feeding and rapid growth. In the pupa stage no food is taken and there is no growth in size, but rapid development takes place. In the perfect stage food is eaten, but no growth in size takes place. In this stage the eggs are produced. When there is very little resemblance between the larva and the imago, and no pupal stage, the metamorphosis, or change, is said to be complete. When, as with the grasshopper, no very marked change takes place between the larva and the imago, there being no pupal stage, the metamorphosis is said to be incomplete. By studying the illustrations and specimens, and by thinking of your past observations of insects, determine which of the insects in the following list have a complete metamorphosis: beetle, house fly, grasshopper, butterfly, cricket, wasp.
Fig. 138.—The Four Stages of a Botfly, all enlarged.
a, egg on hair of horse (bitten off and swallowed); b, larva; c, larva with hooks for holding to lining of stomach; d, pupal stage, passed in the earth; e, adult horse fly.