272 MOTIONS OF INSECTS, 



As a further lielp, others again call in the assistance 

 of their unguil'orni mandibles. Tiiese, which are peculiar 

 to grubs with a variable membranaceous, or rather re- 

 tractile, head ^, especially those of the fly tribe (iV/?/5c/(/^), 

 when the animal does not use them, are retracted not only 

 within the head, but even within the segments behind it** ; 

 but when it is movmg they are protruded, and lay hold of 

 the surface on which it is placed. They were long ago 

 noticed by the accurate Ray. " This blackness in the 

 head," says he, speaking of the maggot of the common 

 flesh-fly, " is caused by two black spines or hooks, 

 which when in motion it puts forth, and fixing them 

 in the ground, so drags along its body." — The larvae 

 of the aphidivorous flies (Si/rp/ms, &c.), the ravages of 

 which amongst the Aphides I have before described to 

 you^, transport themselves from place to place in the 

 same way, walking by means of their teeth. Fixing 

 their hind part to the substances on whicli they are 

 moving, they give their body its greatest possible ten- 

 sion ; and, if I may so speak, thus take as long a step 

 as they can : next, laying hold of it with their mandi- 

 bles, by setting free the tail and relaxing the tension, 

 the former is brought near the head. Thus the animal 

 proceeds, and thus will even walk upon glass '^. Some 

 grubs, as the lesser house-fly {Anthomyia caiiicularis)^ 

 have only one of these claw-teeth ; and in some they have 

 the form as well as the office of legs ^ Bonnet mentions 

 an apodous larva, that, before it can use its mandibles, 



» See MacLeay in Philos. Mag. ^c. N. Ser. No. 9. 178- 

 " De Geer, vi. 65. ' Hist. Ins. 2/0. 



•' Vol. I. 265. " Reaumur, iii. 369. 



f Vol. I. 137. Do Geer, vi. 76. Reaumur, iv. .'576. Swamm. 

 Bihl. yat. Ed. Hill, ii. 46. a. /. xxxix./. 3, /t. h. 



