J. Rennie and C. H. Sutherland 207 
plus agees, en croissance active, le kyste, detruit anterieurement, n’abrite 
plus que la region posterieure; le calice chitineux devenu plus epais emboite 
etroitement 1 extremite postanale du parasite, allongee en un court siphon 
respiratoire bisegmente... .La structure histologique de ces organes permet en 
effet d affirmer leur nature tracheenne. A la base, les cellules hypodermiques 
sont abondamment developpees et en plusieurs couches. La secretion chiti- 
neuse ne forme plus, par suite, un simple filament spiral, mais une couche 
continue, epaisse et noire, de chitine: c’est cette region qui constitue propre- 
ment le calice. Anterieurement, l’epaisseur de la paroi kystale s’attenue, 
comme par etirement de la formation precedente, jusqu’a se reduire a une 
mince couche chitineuse incolore oil Ton ne distingue plus .que quelques ilots 
de cellules hypodermiques, les debris de mues s’ajoutent a l’ensemble.” 
In Bucentes geniculata we have found structural relations between host 
and parasite similar to those cited above (PL XIV, figs. 1, 3, 4, 5, 6, 7 and 8). 
The larva of B. geniculata lives in the body cavity of Tipula paludosa. 
We have not, so far, found it free in this situation, but always attached to 
one of the main tracheal trunks of its host by means of a chitinous sheath-like 
structure similar to that described by the observers quoted. 
At its junction with the trachea of the host, and for a short distance along 
its length, this sheath is thicker and of a dark brown colour. Beyond this, it 
is membranous in appearance and completely encloses the parasite. Usually, 
however, and more particularly in the older larvae, the head end of the parasite 
is extruded from the sheath freely into the body cavity of the host. 
Recalling Pantel’s description and figures of the genesis of the funnel, in 
which he shows that the hooks at the posterior end of the parasite are utilised 
to perforate the skin of the host, it seems probable to us that the attachment 
between parasite and the tracheal system is established in a similar manner 
in Bucentes geniculata. The body spines of the first-stage larva appear well 
• suited for such a purpose. At first the hind end of the parasite is adherent to 
the host at the place where the inflammatory reaction is developed, and its skin 
becomes incorporated with the funnel. On moulting taking place, the larva 
leaves this portion of its cuticle and retracts away, leaving a clear space in 
this area. The relation thus set up between parasite and host involves the 
perforation of the host s tracheal system and establishes a common respiratorv 
system for both. Within the sheath there are always to be found the mouth- 
parts and spiracles of the previous moults. In this way we have obtained 
from third-stage larvae two sets of moulted mouth-parts and both anterior 
and posterior spiracles. 
Description of Larva. 
First-stage Larva. As in almost all Cyclorraphous Diptera, the larva 
passes through three stages separated by two moults. The first-stage larvae 
obtained measured about 1*5 mm. Thirteen segments could be distinguished. 
The head, as is usual in Cyclorraphous Diptera, is divided anteriorly by 
Parasitology xii m 
