MOTIONS OF INSECTS. 459 



Ray tells us in the place just quoted, pushes itself by the protruded spines 

 of its tail. The larva, also, of a long-legged gnat {Limnobia replicata), 

 which in that state lives in the water, is furnished with these anal claws, 

 which, in conjunction with its annular tension and relaxation, and the 

 hooks of its mouth, assist it in walking over the aquatic plants.^ 



A remarkable difference, according to their station, obtains in the bots 

 of gad-flies (^(Estndce) ; those that are subcutaneous (Cuticola Clark) 

 having no unguiform mandibles ; while those that are gastric (GostricoJcE 

 Clark), and those that inhabit the maxillary sinuses of animals (Cavicolce 

 Clark), are furnished with them. In this we evidently see Creative Wis- 

 dom adapting means to their end, for the cuticular bots having no plane 

 surface to move upon, and imbibing a liquid food, in them the mandibular 

 hooks would be superfluous. But they are furnished with other means 

 by which ihey can accomplish such motions, and in contrary directions, 

 as are necessary to them ; the anterior part of each segment being beset 

 with numbers of very minute spines, not visible except under a strono- 

 magnifier, sometimes arranged in bundles, which all look towards the 

 anus ; and the posterior part is, as it were, paved with similar hooks, but 

 smaller, which point to the head. Thus we may conceive, when the 

 animal wants to move forward, that it pushes itself by the first set of 

 hooks, keeping the rest, which would otherwise impede motion in that 

 direction, pressed close to its skin, or it may depress that part of the seor- 

 ment, and when it would move backwards that it employs the second.^ 

 The other descriptions of bots, not being embedded in the flesh, but fixed 

 to a plane, are armed with the mandibles in question, by which they can 

 not only suspend themselves in their several stations, but likewise, with 

 the aid of the spines with which their segments also are furnished, move 

 at their pleasure.^ Other larvae of flies, as well as the bots, are furnished 

 with spines or hooks — by which they take stronger hold — to assist them 

 in their motions. Those mentioned in my last letter as inhabitino- the 

 nests of humble-bees, besides the six radii that arm their anus, and which 

 perhaps, may assist them in locomotion, have the margin of their body 

 fringed with a double row of short spines, which are, doubtless, useful in 

 the same way. 



The next order of walkers amongst apodous larvae are those that move 

 by means of fleshy tuberculiform or pediform prominences, — which last 

 resemble the spurious legs of the caterpillars of most Lcpidoptera. Some, 

 a kind of monopods, have only one of such prominences, which bein^ 

 always fixed almost under the head, may serve, in some degree, the pur- 

 pose of an unguiform mandible. The grub of a kind of gnat (Chiro- 

 nomus stercorarius), and also another, probably of the Tipularian tribe 

 (found by De Geer in a subputrescent stalk of Angelica, which he was 

 unable to trace to the fly), have each a fleshy leg on the underside of the 

 first segment, which points towards the head and assists them in their 

 motions.'* Others again go a little further, and are supported at their 



> De Geer, vi. 355, 



« Reaurn. iv. 41ti. t. xxxvi. f. 5. Comp. Clark On the Bots, l^cc. 48. 



' Mr. Clark (ibid. 62.) observed only roiin;h points on the bois of the sheep, but the.se also 

 have spines or hooks looking towards the anus, Reaum, iv. 556. t. ixxv. f, II. 13. 15. I 

 also observed them myself in the same grub. 



* De Geer, vi. t, xxii. f, 15. i, t. iviii. f. 8. p. 



40 



