TWO-WINGED INSECTS 375 



ORDER IV. : TWO-WINGS (DIPTERA). 



Mouth parts piercing and suctorial, forming an unsegmented trunk 

 or proboscis ; all the thoracic somites coalescent (separate only in the 

 wingless fleas) ; fore-wings membranous, with few nervures ; hind- wings 

 metamorphosed into club-shaped " balancers," or " halteres." Meta- 

 morphosis complete. 



Sub -Order i: Flies (Brachycera). 



The House Fly (Musca domestica). 



(Length i to ^ inch.) 



A. An Inmate of the House. 

 This insect, as its name implies, is an inmate of the house, where, 

 however, no one welcomes its presence. Late in summer, especially 

 when the nights begin to get cool, the flies come into our houses in such 

 quantities as to become a veritable plague. They soil everything, pilfer 

 our food, and annoy and disturb us in our sleep, however cften we may 

 chase them away. With the approach of winter most of them die. 

 Their ranks are especially apt to be thinned by a fungoid disease (the 

 insects attacked by it appearing as if covered with mould). Only a few 

 survive the winter to continue the species. 



B. Colour, 



The ground colour is a dirty gray. On the thorax there are four 

 black stripes ; the abdomen is of a chequered black above, pale yellow 

 below. 



C. Motion. 



On warm bright days the fly is in constant motion. 



(a\ Jts membranous wings easily carry it to any spot it fancies. Of 

 the two pairs of wings usually found in insects, only the front pair is 

 fully developed. The hind pair is reduced to two minute structures, 

 each covered with a small cutaneous scale. They are described as 

 " balancers," " poisers," or " halteres," and in appearance are not unlike 

 drumsticks. The buzzing noise heard during flight, and the sound 

 (voice) which the insect emits, sometimes voluntarily and also when 

 held fast, are produced in the same manner as in the bee (see p. 364, 

 Section 2, a). 



(b) As in all insects which are good fliers (see white cabbage butterfly 

 and honey-bee), all the three thoracic somites are coalescent. 



