Egg-burster of Eucephalous Fly-larve. 375 
effected by a pair of muscles which are attached to the inside 
of the disc and pass across to the ventral side of the head. 
‘These muscles are easily seen in the perfectly transparent 
larva of Chaoborus (Corethra) ; they are shown also in the 
accompanying diagram of the egg-burster of Theobaldia 
annulata. Though they almost meet on the chitinous disc, 
the muscles are wide apart at their ventral insertion. I have 
not ascertained whether they persist beyond the first larval 
stave. One must suppose that the egg-burster is raised and 
so brought into use by blood-pressure on the relaxation of 
this pair of muscles. 
The precise form of the chitinous dise varies somewhat 
in the different species examined. It is relatively larger 
and more conspicuous in the Culicide than in the Chirono- 
mide and Mycetophilide, while among the Culicidee it is 
Fig. 4. 
Simulium austent, Edw. 
Dorsal view of head of newly hatched larva, showing minute ege-burster 
near posterior end of clypeus. x about 60. From a spirit- 
specimen, 
decidedly more heavily chitinised in Aédes than it is in 
Theobaldia or Anopheles. In Aédes it is connected by a 
narrow chitinous rod with the posterior part of the clypeus, 
the rod arising from a thickened bar in the chitin. I have 
not been able to detect this rod in other Culicide or in 
the other families examined. ‘The eggs of Aédes have 
usually great powers of resistance to dessication, and pro- 
bably on this account have a thick chorion, which necessitates 
a more efficient egg-burster. 
The two species of Aédes examined show slight differences : 
the egg-burster in A. argenteus is nearer the posterior end 
25* 
