8 INVERTEBRATE ZOOLOGY 



with the very extensive musculature of the legs and wings. The 

 three thoracic somites are of unequal size. The middle one is 

 the largest and bears the wings. Note that the hinder margin 

 of the basal portion of the wing is divided into three prominent 

 lobes. The posterior thoracic somite is the smallest and bears the 

 balancers, which are the morphological equivalents of the second 

 pair of wings, possessed by most insects. These are a pair of 

 minute white, knobbed organs, which project backward from the 

 posterior wall of the somite, each one being covered by the basal 

 lobe of the wing on that side. They have a sensory function. 



The abdomen is composed of eight somites in the male fly and 

 nine in the female. Of these, however, four somites are much 

 larger than the others, and make up the greater part of the 

 abdomen. The sixth, seventh, and eighth in the male are very 

 small and rudimentary. In the female the posterior four form 

 a long, tubular ovipositor, which is usually telescoped into the 

 abdomen but can often be squeezed out by a little pressure. 

 Each of the five anterior abdominal somites has a pair of spiracles. 

 Find them. 



Exercise l. Draw an outline of the dorsal aspect of the fly on a 

 scale of about 10, indicating the segmentation and the parts 

 observed, including the venation of the wings. Label all 

 the parts observed. 



Exercise 2. Turn the fly over on its back and draw one of its legs 

 on a large scale. The names of the different segments of the 

 leg may be obtained from Exercise 3 on page 4. Note, 

 between the two claws on each foot, the two pulvilli — the 

 hairy adhesive pads by means of whose sticky secretions 

 the fly can walk on an inverted surface. 



Exercise 3. Draw, on a large scale, a side view of the head with 

 the proboscis extended. Note carefully the form of the 

 antennae and of the proboscis. The latter is homologous to 

 the under lip or labium of other insects. 



