INSECTS AND HUMAN DISEASE 103 



their tails are more extended, and they exhibit silvery 

 markings, due, in reality, to air beneath the nymphal skins. 

 When they stretch their tails out in a straight line, or 

 nearly so, the skin splits down the back and the perfect 

 insect emerges (figs. 27 and 28). Unlike some small flies, 

 the Simulidce or buffalo gnats, for instance, the adult 

 mosquitoes do not at once fly away, but rest for a while 

 on the floating nymphal skin, till their legs and wings are 

 dry. When able to fly, they at once turn their attention 

 to the increase of their kind. In the case of the malaria 

 mosquitoes, it is necessary for the females to have a meal of 

 blood before] they can lay fertile eggs. Observations show 

 that they feed every night, chiefly in the early night, and 

 just before dawn, their stomachs never becoming empty, 

 and, as the length of life of the adult may extend to several 

 months, it is clear that these insects are an intolerable 

 nuisance to mankind, quite apart from their disease-carry- 

 ing propensities. 



The eggs of the yellow fever mosquito (Stegomyia) form 

 an intermediate stage between the ones just described and 

 those of the common gnat. Sometimes, as in the case of 

 Anopheles, they are laid singly, and at other times, as in 

 the case of Culex, they are laid in rafts (fig. 29). Each egg 

 is of an irregularly oval shape, somewhat like a minute 

 hen's egg, and its surface is much corrugated. In the 

 corrugations are entangled numerous air bubbles, which 

 keep the egg afloat. As in the case of Anopheles, these 

 eggs, at times, assume varied patterns on the surface of 



OO ' ! 



the water. The egg rafts of the common gnat are from 

 one-fifth to one-third of an inch in length, and are roughly 

 boat-shaped. If a single egg be detached from the mass, 

 it will be seen that it is devoid of floats, or any other con- 

 trivance to keep it on the surface of the water, a condition 

 of affairs that continues only so long as the eggs are glued 

 together to form the raft. One end of the egg is much 

 thicker than the other, and to this end is attached a trans- 



