374 



Observations on the Turnip Saw- Fly. 



the outside edge : the outer rough leaves are undoubtedly those 

 which best suit her purpose^ but she often selects the leaflets 

 near the base, where the eggs rest more secure, and scarcely ever 

 places them near the upper end. 



It is supposed that the flies live twelve or fourteen days from 

 their birth, but the females die as soon as they have finished lay- 

 ing their eggs ; yet such is the vitality of that sex, that she will 

 not only survive the separation of the head from the trunk, but 

 has been able to walk, run, and attempt to fly, three hours after 

 decapitation. Mr. Marshall had one, standing and dressing its 

 wings for many hours after losing its head, and it actually lived in 

 this state upwards of three days. 



A single female is capable of laying from 250 to 300 eggs, and 

 sometimes she deposits 20 in a single leaf; in five days, or perhaps 

 less, in fine warm weather, the eggs are hatched ; but if the atmo- 

 sphere be chilly or wet, it is six, seven, and even ten or eleven 

 days before the young caterpillars eat their way through the shell 

 with their little jaws ( fig. /) and crawl through the shining and 

 dilated cuticle ; and, their heads at that period being larger in dia- 

 meter than their bodies, they soon extricate themselves with their 

 fore-feet ; this takes them, with intervals of rest, from fifteen to 

 twenty minutes. When they first emerge from the shell they are 

 scarcely visible, being only about the tenth part of an inch long ; 

 at that time they are nearly white, excepting two dots on the head, 

 but soon become of a dull semitransparent greenish- white colour, 

 the head jet black and shining. In less than two minutes they 

 begin to feed upon the tender underside of the leaves so voraci- 

 ously, that in a few hours these are often drilled through. At this 

 period it is difficult to shake them off, so closely do they cling to 

 a leaf; but when they are about six or seven days old they cast 

 their first skin, and then they are easily dislodged : they have now 

 doubled their length, being one-fifth of an inch long; some of 

 them, probably the female caterpillars, are much longer, and 

 almost jet black, a stripe on the side of the belly being considera- 

 bly paler, and at first the head is whitish, with two black dots. 

 They are at this time very voracious, and may be traced by their 

 large green pellets of dung ; and, having fed for some time, their 

 skins Mdll no longer expand to the extent required, when each 

 caterpillar again fixes its membranous legs, especially the hinder 

 pair, to a leaf or the denuded fibre, and, bursting the seam behind 

 the head, the caterpillar crawls out, leaving its skin attached to 

 the object it stood upon : it has now greatly increased in size, and 

 consequently its ravages are much more evident, and it soon has 

 to cast its skin again ; they now have a more transparent but 

 wrinkled skin, and are of a slate or grey colour, with a pale line 

 along the sides, the underside being pale also, but the head is 



