GRADER'S ORGAN : THE PUPA 301 



ordinary manner ; they are not, however, simple tracheae, but are rather 

 of the nature of air sacs, as their diameter in most species is distinctly 

 greater than would suffice for their main function of conveying air 

 to the tissues, and contracts suddenly, without the giving off of any 

 branches, at the anterior end. In some species such dilatations are 

 not present. 



At the posterior end of the larva of Tabanus there is a curious struc- 

 ture, the function of which has not been determined. It consists of a 



small pear-shaped sac, the broadest end of which is 



. . Graber's organ 



anterior ; the posterior end narrows down to a tine 



tubule, which opens on the integument of the body between the last 

 and the penultimate segment. Within this sac there is a series of 

 capsules set one behind the other in the long axis, and within each of 

 these capsules a pair of small black pyriform bodies, each attached 

 to the anterior side of the capsule by a delicate pedicle. These bodies 

 diminish in size from the anterior end, the first being considerably larger 

 than the rest. They are easily seen through the integument in the 

 living larva. Graber believed that the structure is a sense organ, but 

 Berlese does not consider that this has been proved to be the case. 

 It is well supplied with nerves. The outer sac is an invagination of 

 the integument, and as such has a chitinous lining. 



The pupa is sub-cylindrical, abruptly pointed anteriorly, and tapering 

 somewhat posteriorly ; it is generally yellowish brown to ferruginous 

 brown, finely wrinkled, and has a lateral tuft of hairs 

 on each abdominal segment. On either side of the XU| f jl 9 a and 14 

 head are the antennal sheaths, pointing outwards, and 

 on each side of the median line two large tubercles, each with a central 

 hair ; below these there are two raised areas with sharp edges, separated 

 by a deep ridge. Lower still there is a pair of elevations, also with 

 raised edges, ahd on the ventral surface of the head one or more tuber- 

 cles. The segments of the thorax are indistinct ; the mesothorax bears 

 the large, raised ear-shaped spiracles. The abdominal segments are 

 free and about equal in length, and have one or more hairs near their 

 hind margins ; the second to the seventh segments inclusive have well- 

 marked lateral ridges, covered with long hairs which are continued into 

 the dorsal and ventral surfaces. These hairs increase in length from 

 before backwards, and are best developed on the seventh segment. The 

 eighth segment is short, and is armed with six projecting spurs or teeth, 

 and with a large anal tubercle. In the male the tubercle is ribbed and 

 bounded anteriorly by a continuous fringe of strong spines ; in the female 



