(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.
Fig. 220.—Cockroach, male, seen from above. (× 1.)