32 Descriptive Zoology. 



The long, coiled sucking tube, composed of the two max- 

 illae, is a noticeable feature. The legs are small and weak, 

 some forms having but two pairs, others having the 'anterior 

 pair but not using them. 



ORDER DIPTERA. 



The House Fly. The house fly has but one pair of 

 developed wings, the second pair being represented by a 

 pair of bodies resembling pins, that is, consisting of a 

 threadlike stalk with a knob at the end. They are called 

 balancers ; their function is supposed to be sensory. When 

 the fly is at rest the wings are extended backward and 

 held horizontally over the back, lapping over each other at 

 the inner borders, but are not folded, in the strict sense of 

 that term; that is, are not thrown into folds, as are the 

 inner wings of the grasshopper. 



The mandibles and maxillae are rudimentary, and the 

 proboscis is composed mainly of the labial palps, which 

 are developed into broad plates, which are thus adapted 

 not only for lapping but also for rasping. They cannot 

 bite, though they often light on the human skin to lap up 

 the perspiration. 



The wing movements are very rapid, making as many as 

 330 vibrations in a second. The sound produced by flies 

 is mainly made by the vibrations of the wings; but when 

 the fly is held by the wings, there is still heard a faint 

 buzzing noise, and this is supposed to be due to the pas- 

 sage of air through the spiracles. 



Development of the House Fly. House flies lay their 

 eggs in stable manure, each female laying about 150 eggs. 

 In favorable weather, the eggs hatch in about one day. 



