Air, Clark'j Ohferviitions on the Genus Oejlrus. 305 



In corroboration of this we may obfcrve, that while the refpi- 

 ratory organs are i'o large in the larva^ they arc remarkably finall 

 in the perfect inie6t, which aU'o, in general, has occafion for very 

 little food. 



Perhaps the fuperior fize of the air vefTels of the bot, compared 

 with the larvcv of other infects, arifes from the greater rarefaction 

 and impurity of the air it is expofed to in the ftomach, which may 

 render a larger portion of it iiccelTary. The remaining undecom- 

 pofed air in the air tubes appears to pafs out by means of the fpira- 

 cula principally, and alfo perhaps by the two horny points obferv- 

 ablc on the firft fcgment. See fig. 22, a. 



Upon this fubjeiit it may not be improper to notice the air veilels 

 of the larva of the Mufca penduhiy which are conftrucled in a very 

 diiFerent way from any others I have feen. The two principal trunks- 

 in' this larva are made up of femicircular cartilaginous rings or 

 fibres, which are difpofed in a fpiral dire6tion, fo as to form the 

 tube. It is evident by this ftructure, that the area of the tube 

 may be entirely obliterated, and the fides be brought into conta(5t. 



The convenience attending this ftru<3:ure, to a larva living in 

 putrid fluids of confiderable depth, appears to be, that befide its ufe 

 in refpiration, it may ferve the fame office as the air bladder in 

 fillies, regulating by its contraftion, or expanfion, the denfity or 

 rarity of the included air, and confequently the defcent or afcent of 

 the larva in thofe fluids. 



The larva of the CE. Equi attain their full growth about the 

 latter end of May, and are coming from the horfe from this time 

 to the latter end of June, or fometimes later. On dropping to the 

 ground they find out fome convenient retreat, and change to the 

 cbryfalis ; and in about fix or feven weeks the fly appears. 



Though this is by far the moft common fpecies of the genus, I 



havfc 



