MOSQUITOES AND FLIES 



Within the puparium, the larva sheds another skin, and 

 then transforms to the pupa. The pupa (Fig. 182 F) is 

 thus protected during its transformation to the adult by 

 the puparial skin of the larva, which serves in place of a 

 cocoon. When the adult is fully formed, it pushes off a 

 circular cap from the anterior end of its case, and the fly 

 emerges. The length ot the entire cycle from egg to adult 

 varies according to temperature conditions, but it is 

 usually from twelve to fourteen days. The adult flies are 

 short-lived in summer, thirty days, or not more than two 

 months, being their usual span of life. In cooler weather, 

 however, when their activities are suppressed, they live 

 longer, and a few survive the winter in protected places. 



One of the essential differences between flies of the 

 house fly type and the mosquitoes and horseflies is in the 

 structure of the mouth parts. The house fly lacks mandi- 

 bles and maxillae, but it retains the median members of 

 the normal group of mouth-part pieces, which are the 

 labrum, the hypopharynx, and the labium. These parts 

 are combined to form a sucking proboscis that is ordi- 

 narily folded beneath the head, but which is extended 

 downward when in use (Fig. 183 A, Prb). 



The labium (Fig. 183 B, Lb) is the principal component 

 ot the proboscis of the house fly, and its terminal lobes, or 

 labella {La), are particularly well developed. From the 

 base ot the labium there projects forward a pair of palps 

 (Pip), which are probably the palpi of the maxillae, 

 though those organs are otherwise lacking. The anterior 

 surface of the labium is deeply concave, but its trough- 

 like hollow is closed by the labrum (Lm). Against the 

 labial wall of the inclosed channel lies the hypopharynx 

 (Hphy). When the lobes of the labium are spread out, the 

 anterior cleft between them is closed except for a small 

 central aperture {a). This opening becomes the func- 

 tional mouth of the fly, though the true mouth is situated, 

 as in other insects, between the bases of the labrum and 

 the hypopharynx, and opens into a large sucking pump 



[345] 



