DEVELOPMENT OF TRICHOGRAMMA EVANESCENS. 153 
happens in development when two eggs are laid in the same 
Donacia egg I do not know, but one generally finds the two 
eggs in different stages of development. Probably the older 
embryo succeeds in the end in killing the other, for I have not 
yet found more than one insect imerging from one egg. 
I am unable to say whether there is more than one brood 
of parasites during the summer, but it is possible to collect 
at the same time eggs containing parasites ready to emerge, 
and some containing newly laid eggs. This points to there 
being more than one brood. There are two or three species 
of Donacia fairly common at Oxford, and they appear one 
after the other, so that this strengthens the view that several 
broods occur in one season. During the winter months I 
have not found the empty egg-cases of Donacia on the stems 
of the reeds, and I have not been able to satisfy myself as to 
whether the parasite hibernate in the eg’g-cases or whether 
they emerge in summer and creep into crevices with a view to 
wintering there. Nearly every year the reeds upon which the 
egg-masses are laid are submerged in the floods, and become 
withered and torn, and thoroughly soaked. For this reason 
it is unlikely that the parasites would remain in the eggs 
which they have destroyed. 
In remarking on the parasite and its host, I do not over- 
look the possibility of T. evanescens being found on the 
eggs of other insects . 1 
Technique. 
The egg of Donacia is covered by a thick chorion which, 
added to the yolk, makes sectioning a very difficult business. 
The parasitised egg-masses were generally preserved in 
Petrunchekewitsch, with a little more nitric acid than usual. 
This often gave splendid results. A mixture of Petrunche- 
kewitsch 2 and Bouin 2 was also tried with about equal results. 
1 Prof. Poulton informs me that this Chalcid parasitises the eggs of 
Dragon flies. I have since been able to observe this interesting fact 
myself. 
2 For these fixatives see Bolls Lee's Microtomists’ Vade-Mecum. 
