202 ELEMENTARY ZOOLOGY 



the pair of needle-like mandibles. The mouth-parts of the blowfly 

 are composed almost exclusively of the thick fleshy proboscis-like 

 labium, which is expanded at the tip to form a rasping organ 



The Diptera or true flies are readily distinguishable 

 from other insects by their having a single pair of wings 

 instead of two pairs, the hind wings being transformed 

 into small knob-headed pedicels called balancers or 

 halteres. The flies undergo complete metamorphosis, 

 and their mouth-parts are fitted for piercing and sucking 

 (as in the mosquito) or for rasping and lapping (as in the 

 blowfly). Nearly 50,000 species of flies are known, more 

 than 4,000 being known in North America alone. 



The blowfly (CallipJiora vomitoria) is common in 

 houses, but can be distinguished from the house-fly by its 

 larger size and its steel-blue abdomen. It lays its eggs 

 on decaying meat (or other organic matter) and the white 

 footless larvae (maggots) hatch in about twenty-four 

 hours. They feed voraciously and become full grown in 

 a few days. They then change into pupae which are 

 brown and seed-like, being completely enclosed in a uni- 

 form chitinized case which wholly conceals the form of the 

 developing fly. The house-fly has a life-history and im- 

 mature stages like the blowfly, but its eggs are deposited 

 on manure. 



The mosquito (Culcx sp.) (fig. 67) lays its eggs in a 

 sooty-black little boat-shaped mass which floats lightly on 

 the surface of the water. In a few days the larvae, or 

 " wrigglers," issue and swim about vigorously by bend- 

 ing the body. The head end of the body is much broader 

 than the other, the thoracic segments being markedly 

 larger than the abdominal ones. The head bears a pair 

 of vibrating tufts of hairs, which set up currents of air that 

 bring microscopic organic particles in the water into the 

 wriggler's mouth. At the posterior tip of the body are 

 two projections, one the breathing-tube (the wriggler 



