October, 1882.] ' 97 



ON SOME POINTS IN THE ECONOMY OF ZAR^A FASCIATA. 

 BY J. A. OSBORNE, M.D. 



On the lOtli of September, last year, I found some larvae of a 

 saw-fly feeding on the leaves of snowberry in my garden. By the 

 22nd of the month they had all spun-up, making dark brown (resinous- 

 looking) cocoons, which, at the end of March and beginning of April 

 of the present year, yielded four female flies of Zarcea fasciata. On 

 the 5th of June, following, a fifth female was excluded, besides 

 these there were, May 30th — June 5th, four ichneumon flies of a 

 smaller species ; and on June 29th and July 10th, two of a larger 

 species. All the saw-flies were females and all laid eggs which de- 

 veloped fnrtlienogenetically. I do not know whether any of these 

 eggs would have failed to develop for intrinsic reasons, but mauy 

 perished through unfavourable external conditions. The eggs were 

 laid singly in mines formed beneath the (upper) epidermis of the leaf; 

 and, being inserted at the edge of the leaf, formed a sort of beading 

 round its circumference. As snowberry is not a native plant I offered 

 some of the flies leaves of honeysuckle (belonging to the same Order), 

 and elder, and in these likewise they laid eggs which hatched out 

 in due course ; but the young larvae refused to feed on these plants, 

 though older ones eat honeysuckle freely enough. I had also eggs 

 laid in leaves of osier and oak ; and I have no doubt these flies would 

 oviposit in almost any practicable leaf, and that the eggs would hatch 

 if due conditions of moisture, &c., were maintained. Some eggs that 

 were laid loosely in the pots, and were never in any leaf at all, I reared 

 on pieces of glass by keeping them constantly moistened with pure 

 water in a covered vessel. I likewise repeated the same experiment 

 successfully with the eggs of Nematus rihesil (ventricosiis, KL). The 

 growth of the egg, theu, by which it attains about three times its 

 original size, can only be due to the imbibition of water, and owes 

 nothing to any nutritive juices derived from the plant. In the case 

 of Nematus, indeed, it is hard to see how anything but moisture 

 could be derived from the plant, as no incision in the leaf is made, 

 and the eggs are simply deposited on the under-surface of the nerves. 

 After four moults the larvae spun up among surface-clay and leaves: 

 but although the earliest cocoons were made on 18th May, no flies 

 have been excluded from any of them yet. It is probable, therefore, 

 that the insect is not double-brooded as I at first supposed, and that 

 the development of my last year's cocoons was hastened by their being 

 kept in the house all winter. At present (2ud September) I find the 



