DIPTERA. 421 



holes in the sides of its head, hke those of certain water-bugs 

 {Ncpa and Belostoma), and somewhat resemble them in shape ; 

 while the transformations of the flea are not very much unlike 

 those of the flies, whose maggots cast off" their skins on becoming 

 pupae. 



The foregoing remarks, on the structure, habits, and transfor- 

 mations of Dipterous insects, have been somewhat extended be- 

 yond what was originally designed, in order to make amends for the 

 want of information relating to the history of the gnats and flies 

 which are injurious to vegetation. The consideration of the lat- 

 ter has been reserved to the last, and the printing of this portion 

 of my essay has been delayed, in the hope that additional and 

 more valuable materials would be obtained. Tt is a source of re- 

 gret to me that I have nothing original to offer on this subject, 

 not having had an opportunity to watch the growth and changes 

 of these destructive insects myself. Very little success has at- 

 tended my efforts to collect new materials for the history of the 

 insects that are injurious to wheat and other cultivated grains ; 

 and my numerous private letters and public communications, so- 

 liciting information, have procured for me only a few facts in an- 

 swer to them. In looking over a great number of volumes for 

 papers relative to these insects, my labors have been rewarded, 

 as it were, with only a few scattered grains among bushels of chaff". 



The far-famed Hessian fly and the wheat-fly of Europe, and 

 of this country, are small gnats or midges, and belong to the 

 family called Cecidomyiad^, or gall-gnats. The insects of this 

 family are very numerous, and most of them, in the maggot 

 state, live in galls or unnatural enlargements of the stems, leaves, 

 and buds of plants, caused by the punctures of the winged in- 

 sects in laying their eggs. The Hessian fly, wheat-fly, and 

 some others differ from the majority in not producing such altera- 

 tions in plants. The proboscis of these insects is very short, and 

 does not contain the piercing bristles found in the long proboscis 

 of the biting gnats and mosquitos. Their antennae are long, com- 

 posed of many little, bead-like joints, which are larger in the 

 males than in the other sex ; and each joint is surrounded with 

 short hairs. Their eyes are kidney-shaped. Their legs are 

 rather long and very slender. Their wings have only two, three. 



