22 NATURAL HISTORY OF AQUATIC INSECTS 



a time. Again, the many Insects which frequent 

 small pools, liable to disappear in summer drought, 

 will enjoy a great advantage if they are able to fly 

 abroad, and lay eggs in places which offer a supply 

 of food and yet are not permanently habitable. 

 Where these considerations do not apply, as for \ 

 instance in the case of Moths whose food-plants 

 (grass, heather, &c.) are social and not easily ex- 

 hausted, or of Moths whose larvae are indiscriminate 

 in their food (the Vapourer is a good example) we 

 often find that the female is wingless.^ 



The mating of the male with the female is another 

 motive for the acquisition of wings. Were the fully 

 developed Insect restricted, like the larva, to one pool 

 or to one food-plant, choice of mate and mixture of 

 families would be hard to obtain. Hence even in 

 those cases (wingless Moths, Scale-insects, Strepsip- 

 tera) where abundance of food or parasitic life has 

 led to the suppression of the wings of the female, and 

 where the eggs are not widely dispersed, the male 

 Insect is nearly always capable of flight. 



Though no Insect is so completely aquatic in its 

 final stage as to be unable to live in the air, we do 

 find, as a rare exception, that the final stage is sup- 

 pressed, and that the pupa becomes capable of laying 

 fertile, though unfertilised eggs. This is the case 

 with Chironomus. Grimm - found in his aquarium a 

 small species which in spring and summer laid un- 



^ Weismann, Biological Memoirs. Essay on the " Duration 

 of Life." 



^ " Die ungeschlechtl. Fortplantzung einer Chironomus- 

 Art." Mdm. Ac. St. Petersb. VII. Ser., Tom. XV. (1870). 



