Elementary Zoology, Second Edition

by Vernon L. Kellogg

Fig. 38.—The red-legged locust, Melanoplus femur-rubrum, to show external structure.

Note the smooth, rather firm and horny character of the body. This is due to the fact that the skin is everywhere covered with a cuticle in which is deposited a horny substance called chitin. The cuticle is not uniformly firm over the body. At the junction of the body segments in the abdomen, in the neck and between the segments of the legs, in fact, wherever motion is desirable, the cuticle is flexible, thus making bending of the body-wall possible. Elsewhere, however, it is hard and stiff, serving not only as a protective coat or armor over the body, but also affording firm places for the attachment of muscles.

Insects (and all other Arthropods) have no[9] internal skeleton, but, in this firm cuticle, an exoskeleton.

Although the head is apparently a single segment, it is really composed of six or seven body segments greatly modified and firmly fused together. Note that it bears a pair of large compound eyes and three much smaller simple eyes or ocelli.

Technical Note.—Strip off a bit of the outer covering of a compound eye, mount on a glass slide and examine under the microscope.

Note that, as in the crayfish, each compound eye is composed externally of many small hexagonal facets, the outer covering, the cornea, being simply the cuticular covering of the body, in this place transparent and divided into small facets. Besides the eyes, the head bears also several movable appendages, namely the antennæ, and the mouth-parts. Note the number, place of insertion, and segmented character of the antennæ. These antennæ are sense-organs and are used for feeling, smelling, and, in some insects, for hearing. Note that the mouth-parts consist of an upper, broad, flap-like piece, the[10]labrum; of a pair of brown, strongly chitinized, toothed jaws or mandibles; of a second pair of jaw-like structures, the maxillæ, each of which is composed of several parts; and of an under, freely-movable flap, the labium, also composed of several pieces. Each maxilla bears a slender feeler or palpus composed of five segments. The labium bears a pair of similar palpi, which are, however, only three-segmented. The mandibles and maxillæ, which are the insect jaws, move laterally, not vertically as with most animals.

Make drawings of the lateral aspect of the head; of a bit of the cornea; of the dissected out mouth-parts.

Of the three segments of the thoracic region of the body, the most anterior one is called the prothorax. It is freely movable and has a large hood or saddle-shaped piece, the pronotum, on its dorsal aspect, and a blunt-pointed tubercle on the ventral aspect. The foremost pair of legs is attached to the prothorax. The next segment is the mesothorax, which is immovably fused to the next thoracic segment. What appendages does it bear? The third segment is the metathorax, which besides being fused with the mesothorax in front, is similarly fused with the foremost abdominal segment behind. What appendages does the metathorax bear?

Examine one of the fore legs and note that it is composed of a series of unequal parts or segments. The segment nearest the body is sub-globular and is called the coxa; the second segment is smaller than the coxa and is called the trochanter; the third, known as the femur, is the largest of all; the fourth, tibia, is long and slender; and the next three, the last of which is the terminal one and bears a pair of claws and between them a little pad, the pulvillus, are called the tarsal segments. Most insects have five tarsal segments. Note the great size of the hindmost or leaping legs. Determine the segments of the middle and hindmost legs. Make a drawing of a fore leg.