Microscopical Essays. 



The head of thelarvaof this infect is exceedingly Angular, being 

 covered with a mafic which goes over the whole fore part of the 

 head, having proper cavities within to fuit the different pro- 

 minencies of the face, and fits it more exactly than the common 

 mafic does the human face ; it is of a triangular form, growing 

 fmaller towards the bottom : at the bottom there is a knuckle 

 which fits a cavity near the neck ; on this part it turns as on a 

 pivot. The upper part of this mafk is divided into two pieces, 

 or fhutters, which the infect can open or clofe at pleafure ; it can 

 alfo let down the whole mafk whenever it pleafes. The edges 

 of the fhutters are toothed like a faw. It makes ufe of the mafk 

 to feize and hold it's prey. 



There is a confiderable difference in the fhape of thefe mafks in 

 different fpecies of the libellula, fome having two claws near the 

 top of it, which they can throw in or out at pleafure, and which 

 make it a very formidable inflrument to the infects on which it 

 feeds. 



Thefe animals generally live and feed at the bottom of the 

 water, fwimming only occafionally ; their manner of fwimming, 

 or rather moving in the water, is curious, being by fudden jerks 

 given at intervals ; but this motion is not occafioned by their 

 legs, which at this time are kept immoveable and clofe to the 

 body : it is by forcing Out a ftream of water from the tail, that 

 the body is carried forwards ; this may be eafily ' perceived by 

 placing them in a flat veffel, in which there is only jufl water 

 enough to cover the bottom. Here the action of the water fquirted 

 from their tail will be very vifible ; it will occafion afmall current, 

 and give a fenfible motion to any light bodies that are lying on 



