Microscopical Essays. 227 



After a certain period they pafs into the pupa form ; when 

 they are about to change, they betake themfelves to the herbs 

 that float on the furface of the water, and creep gently thereon, 

 till at length they lie partly on the dry furface, and partly on the 

 water ; when in the larva or pupa ft ate, they can live in water, but 

 can by no means inhabit there when changed into flies : indeed, 

 man alio, whilft in the uterus, lives in water, which he cannot do 

 afterwards. When thefe worms have found a proper fituation, 

 they by degrees contract themfelves, and, in a manner fcarce per- 

 ceivable, lofe all power of moving, The inward parts of the 

 worm's tail now feparate from the outmoft {kin, and become 

 greatly contracted ; this probably gives the infect a great deal of 

 pain ; by this contraction, an empty fpace is left in the exterior 



fkin, into which the air foon penetrates- 



i 



Thus this infect paries into the pupa ftate under it's own fkin, 

 entirely different from that of the caterpillar, which calls off the 

 exterior (kin at this time ; this change may often be obferved to 

 take place in the fpace of ten or twelve hours, but in what manner 

 it is performed we are ignorant, as it is effected in a hidden un- 

 known way, inwardly within the fkin, which conceals it from our 

 "dew. 



Whilft the larva is changing under the fkin, the body, head 3 

 and tail, feparate infenfibly from their outward vefture. The 

 legs at this time, and their cartilaginous bones, are, on account 

 of the parts which are withdrawn from them, left empty; the 

 worm lofes alfo now the former fkull, the beak, together with 

 the horny bones belonging thereto, which remain in the fkin of 

 the exuvia. It is worthy of notice, that, the optic nerves feparate 



E e 2 alfo- 



