Microscopical Essays. 



221 



When it is arrived at the time for it's pupa ftate, it goes through 

 the change without calling off the fkin of the larva. Lallly, in 

 the imago, or fly ft ate, it would infallibly perifli in the water: 

 the element, which had hitherto fupplied it with life and motion, 

 would now be it's immediate deftruclion. 



This infecl: is characterized by Linnaeus as mufca chameleon. 

 Habitat larva in aquis dulcibus ; mufca fupra aquam obambulare 

 folet. In a former edition of the Fauna Suecica he called it 

 oeftrus aquae ;- but on a more minute examination, he found it was 

 a mufca ; befides, the larvae of all known oeftri are nourifhed in 

 the bodies of animals. 



The larva of this infecl, when viewed by the naked eye, ap- 

 pears to conlift of twelve annular di virions, fee Plate XI. Fig. i ;. 

 by thefe it is feparated into a head, thorax, and abdomen ; but 

 as the flomach and inteftines lie equally in the thorax and abdo- 

 men, it is not eafy to diftinguifh their limits until the infedi 

 approaches the pupa ftate. 



The parts mod worthy of notice, when this infect is viewed by 

 the naked eye, are the tail and fnout. The tail is furnifhed wick 

 an elegant crown or circle of hair b, difpofed quite round it in an 

 annular form ; by means of this the tail is fupported on the fur- 

 face of the water while the worm or larva is moving therein, the 

 body in the mean while hanging towards the bottom ; it will 

 fometimes remain in this fituation for a confiderable time, without 

 the leaft fenfible motion. When it has a mind to fink to the bot- 

 tom by means of it's tail, it generally bends the hairs of that part 

 towards each other in the middle, but much clofer towards the 

 6 extremity - 



