20 INVERTEBRATE ZOOLOGY 
INSECTA 
AN INSECT LARVA. A CATERPILLAR 
Notice that the head, thorax, and abdomen are not set off 
from one another. The body is thus worm-like in form, there 
being almost no specialization of the body-parts. Determine 
how much of the body is thorax and how much abdomen. 
The thorax bears three pairs of jointed legs, each one termi- 
nating in a single hook. The abdomen also bears several pairs 
of legs which are not like those of the thorax. How many are 
there and in what do they differ from the thoracic legs? Find 
and count the spiracles, which are usually easily seen. 
Exercise 1. Draw an outline representing a side view of the 
animal on a scale of from 2 to 6; number the thoracic and 
abdominal segments, show the spiracles, and label all the 
parts. 
Study the head with the aid of-a hand lens. Notice the pair 
of large convex plates which, with the small median triangular 
plate, form the wall of the head. Near the lower edge of each of 
the convex plates are several minute ocelli; count them. On the 
ventral side of the head find the antenne; how many joints are 
there in each? The mouth-parts are between the antenne. The 
labrum is bilobed, and beneath it are the dark-colored mandibles. 
Just back of these are the maxille and the labium, the latter 
being a median, elongated, conical organ between the maxille. 
The external opening of the silk glands is in the labium. 
Exercise 2. Draw a front view of the head on a scale of 7. 
Internal anatomy. With fine scissors make a longitudinal 
incision the length of the animal, in the dorsal integument, a 
