33 



sition of the body-wall, but will merely examine the more 

 important external structures. 



Pin a larva to the cork or beeswax lining of a dissecting 

 dish, with its ventral aspect uppermost, and cover it with 

 water. Make a drawing of the ventral surface. Name the 

 regions and the appendages of the body shown in this view. 

 The long, tapering appendages on the margins of the abdo- 

 men may be termed the lateral filaments ; the tufts of hair — 

 like appendages near the bases of the lateral filaments are 

 tracheal gills ; and caudad of the ninth abdominal segment 

 is a pair of prolegs. These may be termed the anal pro- 

 legs.^ 



Make a drawing of each of the following parts : — 



1. The dorsal aspect of the head, showing the mandibles, 

 the antennae, and the labrum. 



2. One of the tracheal gills. Cut off several of the hair- 

 like branches and mount them in glycerine, using a thin 

 cover-glass. Examine them with a compound microscope, 

 using a high power. The surface of each hair is marked 

 with numerous ridges, which resemble in appearance the fine 

 ridges on the skin of the palm of the human hand. In the 

 center of each hair, and extending nearly its entire length, is 

 a large tube. This is a trachea or air vessel. Tracheae can 

 be distinguished from other vessels by being marked with 

 transverse lines, which occur at regular and very short inter- 

 vals. The intimate structure of the tracheae will be studied 

 later ; but at this point the student should become familiar 

 with the appearance of tracheae, so that when he dissects the 

 insect he can readily distinguish them from other vessels. 

 Branching from this large central trachea are numerous very 



* Many larvae bear upon the abdomen locomotive appendages, which resem- 

 ble legs, and are termed prolegs. This is especially true of caterpillars, which 

 bear from one to five pairs of these appendages. The prolegs are temporary 

 organs, being shed with the skin when the larva transforms to a pupa. 



