42 ENTOMOLOGY. 



duce young from unfertilized eggs. Certain moths, as the 

 silk-worm moth (Bombyx mori) and others, have been 

 known to lay unfertilized eggs from which caterpillars have 

 hatched. 



The most surprising case is that of the larva of a dipterous 

 gall-fly (Miastor), which brings forth numbers of young 

 maggots like itself, the eggs developing in the ovaries of 

 this precocious maggot. The pupa of another fly (Chirono- 

 mus) lays eggs from which the maggots hatch. 



This anomalous mode of reproduction is called partheno- 

 genesis, and fundamentally is only a modification of the 

 mode of producing young by budding which is universal in 

 plants, and is not unusual among the lower branches of 

 the animal kingdom. The object or design in nature, at 

 least in the case of the plant-lice and bark-lice, as well as 

 the gall-flies, is the production of large numbers of indi- 

 viduals by which the perpetuity of the species is maintained. 



Broods or Generations of Insects. Most insects live one 

 year; hatching from the egg early in the summer, they 

 pass through the larval state, and early in the autumn be- 

 come pupae, to appear as imagines for a few days or weeks 

 in the succeeding summer. Many moths and butterflies, 

 however, are double-brooded, and some have even three 

 broods. Papilio djax has in West Virginia four and 

 sometimes five generations a year. There are other insects, 

 such as certain kinds of flies, bugs, beetles, etc., which 

 keep up a constant and irregular succession of broods. On 

 the other hand, the seventeen-year Cicada has a generation 

 only once in seventeen years. 



Cold retards the development of insects, while warmth 

 stimulates it; and insects which are as a rule single-brooded 

 may be artificially forced into having a second brood dur- 

 ing the same season. 



Contagious Diseases of Insects due to Animal and Vege- 

 table Germs.* While many insects, especially the white 



* For the latest resume of this subject see S. A. Forties's article 

 " On the Present State of our Knowledge concerning Contagious 



