DIPTERA. 427 



face of the ground, with the head towards the root of the plant. 

 Having- thus fixed themselves upon the stalk, they become sta- 

 tionary, and never move from the place till their transformations 

 are completed. They do not eat the stalk, neither do they pen- 

 etrate within it, as some persons have supposed, but they lie 

 lengthwise upon its surface, covered by the lower part of the 

 leaves, and are nourished wholly by the sap, which they appear 

 to take by suction. They soon lose their reddish color, turn 

 pale, and will be found to be clouded with whitish spots ; and 

 through their transparent skins a greenish stripe may be seen in 

 the middle of their bodies. As they increase in size, and grow 

 plump and firm, they become imbedded in the side of the stem, 

 by the pressure of rheir bodies upon the growing plant. One 

 maggot thus placed seldom destroys the plant ; but, when two or 

 three are fixed in this manner around the stem, they weaken and 

 impoverish the plant, and cause it to fall down, or to wither and 

 die. They usually come to their full size in five or six weeks, 

 and then measure about three twentieths of an inch in length. 

 Their skin now gradually hardens, becomes brownish, and soon 

 changes to a bright chestnut color. This change usually hap- 

 pens about the first of December, when the insect may be said 

 to enter on the pupa slate, for afier this time it takes no more 

 nourishment. Mr. Herrick informs me, that the brown and 

 leathery skin, within which the maggot has changed to a pupa 

 or chrysalis, is long egg-shaped, smooth, and marked with 

 eleven transverse lines, and measures one eighth of an inch 

 in length. In this form it has been commonly likened to a 

 flax-seed. It appears- then, from the remarks of Dr. Chap- 

 man, Mr. Herrick, and other careful observers, that the maggots 

 of the Hessian fly do not cast ofl:" their skins in order to become 

 pupa;, wherein they differ from the larvae of most other gnats, 

 and agree with those of common flies ; neither do they spin co- 

 coons, as some of the Cecidomyians are supposed to do. Mr. 

 Herrick, in one of his letters, observes, that " the pupa 

 gradually cleaves from the dried skin of the larva, and, in the 

 course of two or three weeks, is wholly detached " from it. 

 Still inclosed within this skin, which thus becomes a kind of co- 

 coon or shell for the pupa, it remains throughout the winter, 



