18 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 l. 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 antennae ; how many joints are 

 there in each? The mouth-parts are between the antennae. The 

 labrum is bilobed, and beneath it are the dark-colored mandibles. 

 Just back of these are the maxillae and the labium, the latter 

 being a median, elongated, conical organ between the maxillae. 

 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 



