2i6 TOPOGRAPHICAL AX ATOMY OF THE 



eyes, the median nerve to the ocelH, the ocelH, the antennal 

 nerves, and the frontal sac ; and a lower portion, continuous 

 with the cavity of the rostrum, which lodges the fulcrum and 

 its muscles, the commencement of the oesophagus, and the 

 nerves and retractor muscles of the proboscis. 



In the adult insect both these cavities contain large air sacs, 

 which occup}' the greater portion of them. Those below the 

 tentorium only communicate with those above by a rete mira- 

 bile of small convoluted, cylindrical tracheal vessels, which lie 

 in its substance and form the greater part of the membrane in 

 front of the brain. The tentorium itself consists of two 

 cuticular laminae, separated behind by the oesophagus and the 

 brain, which, although it projects into the upper part of the 

 head cavity, may perhaps be more properly described as lying 

 in the substance of the tentorium ; the latter forms its mem- 

 branous sheath. As, however, the membrane beneath the 

 brain is almost at the same level as the rest of the tentorium, 

 I have preferred to describe the nerve-centres as situated in the 

 supratentorial cavity. It appears to me that the tentorium 

 is in reality a mere thickening of the walls of the tracheal sacs 

 above and below, where they are in apposition with each 

 other, strengthened by the rete of cylindrical vessels already 

 alluded to. 



In the imago when it first escapes from the pupa, the 

 tracheal sacs, which eventually occupy so large a portion of the 

 head cavity, are very small, and the greater part of the head is 

 occupied by blood. At this period the frontal sac is everted 

 and forms a large protuberance, also filled with blood. 



During the efforts made by the insect to escape from the 

 pupa the frontal sac and head alternately become enlarged with 



Description of Plate XI. 



A median vertical section of an adult male Blow -fly : 



rt, antenna ; cl, cloaca ; c s, chyle stomach ; d v, dorsal vessel \ f, fulcrum ; 

 /j-, frontal sac ; /, intestine; i g, infra-cesopliageal ganglia; w, dorsal muscles 

 {dorsaks) ; nun, leg muscles ; / s, pulmonary sac ; pv, proventriculus : r f, rectal 

 papillce ; s _^, supra oesophageal ganglia ; s s, crop or sucking stomach (//^w^yY^a^) ; 

 / 'J, thoracic ganglion ; ts^, scutellar air sac ; //-', scutal air sac. 



