()8 Observatiufis on Lisecls affecting the Turnip Cro])^. 



ascendancy, which once acquhed is not easily overcome.'^' I have 

 been surprised to see how slio^htlj Aphides are affected by wet ; 

 and I find that the same careful observer of nature tried a few 

 experiments to ascertain how far they could resist the action of 

 water. Mr. W. Curtis first immersed some Aphides attached to 

 a willow-twicr in water for sixteen hours, which did not appear to 

 incommode them in the least, for on being- taken out and placed 

 in the sun, they increased and multiplied shortly afterwards ; but 

 when they were brushed off, they could not so lona^ sustain the 

 effect of water. He afterwards immersed two other sorts of 

 Aphides, which at the expiration of twelve hours were all dead : 

 this difference of power in the vital principle is very remarkable, 

 and not easily explained. Their capability of resisting some 

 g-ases or effluvia is likewise very astonishing: for instance, I took 

 some Aphides from the Southernwood, both winged and apterous, 

 and corked them up in a quill containing a piece of camphor, 

 which produced an atmosphere that killed most insects in an 

 hour, but the Aphides were walking about unaffected, after being 

 confined there twenty- four hours ! 



Drosopiiila-}- and Phytomyza.J — Tiirnip-Ieaf Miners, 



Turnip-leaves are often more or less covered with wdiitish blisters, 

 which are caused by the maggots of two different kinds of flies, 

 both of which belong to the Order Diptera and the Family 

 MusciDTE : it does not appear that they do niuch mischief to the 

 crops, but it may be questionable whether the maggots do not 

 occasionally generate disease in cattle feeding upon the turnip- 

 tops, when their numbers are in excess. The first species, which 

 is called Drosophila, belongs to a group, the larvfc of some living 

 in vinegar and acid beer, many breeding in Boleti ; and others, 

 like the one before us, live upon the parenchyma or pulp of the 

 leaves of various plants : some of the flies are frequently found 

 in cellars, and are also seen running over the v»'indovvS in our 

 houses. 



It is not a little singular that, as far as my observations go, the 

 maggots of the turnip Drosophila form their dwellings so care- 

 fully under the upper cuticle of the leaves that not a trace of 

 them can be seen on the under-side, whilst the liabits of the larvre 

 of the Phytomyzcp are just the reverse : the economy of the former 

 I will now investigate. On the 22nd of last July I saw many 

 leaves of some English turnips disfigured by large pale blisters 

 on various parts of the upper side (fig-. 26) : upon examination I 



* Trans. Linn. Soc. v. 6, p. 84. 



t Curt. Brit. Ent. fol. and pi. 473, Guide Gen. 1334. 



X Curt. Brit. Ent. fol. and fol. pi. 393, Guide Gen. 1348. 



