DURATION OF LARVAL STAGE IN SOME OF THE SESIID^. 95 



if it proved to be three years. The few specimens I have bred 

 have all emerged in July, though Barrett gives the latter part of 

 May and June as its period on the wing. 



As regards T. crabroniforme ; here again I cannot speak with 

 certainty, but I have found the larva still feeding in June, and 

 think it probable that the generally accepted opinion that it feeds 

 nearly two years is correct. 



My experience as regards S. vespiformis is very much more 

 extensive, and I feel that I can state positively that this species, 

 at any rate normally, completes its life-cycle in one year. As is 

 well known, the larvae are to be found under the bark of the 

 stumps of felled oak-trees. These trees are invariably felled in 

 the winter ; the following summer the females of S. vespiformis 

 frequent these stumps, especially those that produce a few 

 shoots, though I have never been able to ascertain exactly where 

 the egg is laid. Larvas are to be obtained throughout the 

 following winter and spring. To make this quite clear, I may 

 say that the stumps of oaks felled in 1907-8 will, if the insect 

 occurs in the neighbourhood, probably contain larvae in the 

 winter of 1908-9, and only in rare instances, when the stump is 

 still alive, will a few larvae be found in the winter of 1909-10. 

 The larvae seem of all sizes in April and May ; indeed, I have 

 found larvas, pupae, and imagines on the same day in June. 

 When I first began to study this species, I thought that the 

 smaller larvae would probably only reach the imago stage in the 

 following year. But the emergence of the imago extends over 

 a long period, viz. from the end of May to the end of August. 

 If a stump be searched, however, in the middle or end of July, 

 no larvae are to be found, only empty and a few full cocoons. I 

 think, therefore, that I am justified in concluding that the life of 

 this species, from ovum to imago, extends for only one year, and 

 that when, in rare instances, larvae are found in the winter of 

 1909-10, in the stump of an oak felled in 1907-8, they are the 

 progeny of imagines that emerged in 1909. It is possible that, 

 as with other species of Lepidoptera, a few pupae, or even larvae, 

 may "lie over" till another year, but I am convinced that the 

 normal period during which the larva feeds is just under one 

 year. I may state that during the past five years I have taken 

 over two hundred larvae and pupae of this species, and have 

 purposely searched for them at different times, with a view to 

 ascertaining the duration of the larval stage. 



I have not had as extensive opportunities of investigation 

 regarding S. culiciformis as with S. vespiformis. But the former 

 species is to be found in the larval stage in the live stumps of 

 birch the second spring after it has been cut, and not, so far as 

 my experience goes, in the third spring ; so I conclude that this 

 species has also a one-year cycle. 



Timworth Hall, Bury St. Edmunds : Feb. 12th, 1911. 



