A FLY 5 



Class : Tracheaia. Division : Insecta. Order : Diptera 

 A FLY 



Kill several bluebottle flies or large house flies, without muti- 

 lating them, and impale one specimen on a slender insect pin or 

 a needle. Stick the pin or needle into a cork or a small piece of 

 wood, in order to be able to handle it easily, and study the exter- 

 nal anatomy of the fly with the aid of a hand lens. 



Observe the compact body of the animal, and note that it is 

 distinctly divided, like that of the wasp, into three divisions: 

 the head, the thorax, and the abdomen. Observe the color and 

 the hairy surface of the body, including the legs and the wings. 

 These numerous hairs are projections of the cuticula, and perform 

 a useful function as tactile organs ; that is, they are sensitive to 

 vibrations of the atmosphere, and thus function as sense organs 

 in that they aid in giving the animal a knowledge of its surround- 

 ings. Note the three pairs of long, strong legs and the single pair 

 of wings. The fly has unusual locomotory powers. Correlated 

 with these powers are the long cuticular hairs just mentioned, and 

 also the very large composite eyes. An active, rapidly moving 

 animal like the fly needs well-developed organs of orientation. 

 The eyes are larger in the male than in the female, and are closer 

 together on the top of the head. The two sexes may thus be 

 distinguished. 



Between the large eyes are the three minute accessory eyes, or 

 ocelli. Note the peculiar form of the small antennae, with their 

 pinnate terminal portion. Extend the proboscis and observe its 

 complex structure and the oral lobes at the lower end. The fly 

 eats only fluid food, which it sucks up through its proboscis. 



The thorax is of relatively large size, being almost entirely filled 

 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 bal- 

 ancers, which are the morphological equivalents of the second 



