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 1. 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. 
Bzercise 3. Draw, on a large scale, a side view of the head with 
the proboscis extended. Note carefully the form of the 
antenna and of the proboscis. The latter is homologous to 
the under lip or labium of other insects. 
