Mr. Clark' j Qbfervatiens en the Genus Ocjlrus, 303 



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

 ratory organs are fo large in the larva, they are remarkably I'm all 

 in the perfect infect, which alio,' in general, has occafion for very 

 little food. 



Perhaps the fuperior fize of the air vcffcls of the hot, compared 

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

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

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

 pofed air in the air tubes appears to pais out by means of the fp'ira- 

 cuhi principally, and alio perhaps by the two horny points oblerv- 

 ablc on the fir ft fegment. See fig. 22, a. 



Upon this fubject it may not be improper to notice the air veiTels 

 of the larva of the Mufca pendula, which arc conftructcd in a very 

 different 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 direction, lo 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 contact. 



The convenience attending this ftructure, to a larva living in 

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

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

 fillies, regulating by its contraction, 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. Equl 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 

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



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



have 



