10 NORTH AMERICAN DIRTERA. 



whicli Winnertz says is produced by an exudation or excretion 

 from tlie larvse and is not spun. Larvae which had fastened 

 themselves to the leaf were surrounded in twenty-four hours 

 by a white halo of thread-like particles like spicules of crystals, 

 the larvae remaining almost wholly motionless. The cocoon 

 is completed within a few days but even then shows no traces 

 of the genuine web. Tliere seems to be no doubt, however, 

 but that they do in some cases actually spin a cocoon, as is 

 so frequently the case among the fungus-gnats. The change 

 to the pupa state is marked by an alteration of color, the ante- 

 rior segments of the larvae become distended, and the legs, 

 wings, and antennae developing and rapidly attaining their 

 full pupal dimensions. 



The period of pupation is very variable, depending upon 

 warmth and moisture. The development is retarded by 

 cold, though they are capable of withstanding a very low de- 

 gree of temperature. 



One of the most remarkable things in the biology of these 

 or any other insects was discovered by Wagner in 1860. Ac- 

 cording to this scientist, the larvae of certain species belonging 

 to the genus Miastor, and which live under the bark of trees, 

 produce from ovary -like organs a number of eggs which hatch 

 within the abdominal cavity of the parent and there remain, 

 feeding upon the abdominal tissues which surround them, 

 until they are consumed. They then escape to increase in 

 size and produce another generation in the same remarkable 

 manner. These series of asexual reproductions are repeated 

 until finally, from the last brood, pupae are developed whicli 

 undergo their transformation into imagines, and from which 

 a new series of eggs, agamic broods of larvae and pupae in 

 their turn result. There are other instances of paedogenetic 

 reproduction among diptera, but none so interesting as 

 this. 



This family contains some of the most destructive of all in- 

 sects in man's economy, and perha})s tlu^ most iniportaut of 



