INSECTS AND ARACHNIDA. 245 



mounted without compression, furnish a peculiarly beautiful object for 

 the Binocular Microscope. The Feet of Caterpillars differ consider- 

 ably from those of perfect Insects. Those of the first three segments, 

 which are afterwards to be replaced by true legs, are furnished with strong 

 horny claws; but each of those of the other segments, which are termed 

 6 pro-legs/ is composed of a circular series of comparatively slender curved 

 hooklets, by which the Caterpillar is enabled to cling to the minute rough- 

 nesses of the surface of the leaves, etc., on which it feeds. This structure 

 is well seen in the pro-legs of the common Silkworm. 



641. Stings and Ovipositors. The insects of the order Hymenoptera 

 are all distinguished by the prolongation of the last segment of the abdo- 

 men into a peculiar organ, which in one division of the order is a * sting/ 

 and in the other is an ' ovipositor ' or instrument for the deposition of 

 the eggs, which is usually also provided with the means of boring a hole 

 for their reception. The former group consists of the Bees, Wasps, Ants, 

 etc.; the latter of the Saw-flies, Gall-flies, Ichneumon-flies, etc. These 

 two sets of instruments are not so unlike in structure, as they are in func- 

 tion. The 'sting' is usually formed of a pair of darts, beset with barbed 

 teeth at their points, and furnished at their roots with powerful muscles, 

 whereby they can be caused to project from their sheath, which is a 

 horny case formed by the prolongation of the integument of the last seg- 

 ment, slit into two halves, which separate to allow the protrusion of the 

 of the sting; whilst the peculiar 'venom' of the sting is due to the ejec- 

 tion, by the same muscular action, of a poisonous liquid, from a bag situ- 

 ated near the root of the sting, which passes down a canal excavated 

 between the darts, so as to be inserted into the puncture which they 

 make. The stings of the common Bee, Wasp, and Hornet, may all be 

 made to display this structure without much difficulty in the dissection. 

 The 'ovipositor' of such insects as deposit their eggs in holes ready- 

 made, or in soft animal or vegetable substances (as is the case with the 

 Ichneumonidce), is simply a long tube, which is inclosed, like the sting, 

 in a cleft sheath. In the Gall-flies ( Cynipidce), the extremity of the ovi- 

 positor has a toothed edge, so as to act as a kind of saw whereby harder 

 substances may be penetrated; and thus an aperture is made in the leaf, 

 stalk, or bud of the plant or tree infested by the particular species, in 

 which the egg is deposited, together with a drop of fluid that has a pecu- 

 liarly irritating effect upon the vegetable tissues, occasioning the produc- 

 tion of the 'galls,' which are new growths that serve not only to protect 

 the larvae, but also to afford them nutriment. The oak is infested by 

 by several species of these Insects, which deposit their eggs in different 

 parts of its fabric; and some of the small 'galls' which are often found 

 upon the surface of oak-leaves, are extremely beautiful objects for the 

 lower powers of the Microscope. In the Tenthredinidto, or 'saw-flies,' 

 and in their allies the Siricidcv, the ovipositor is furnished with a still 

 more powerful apparatus for penetration, by means of which some of 

 these Insects can bore into hard timber. This consists 'of a pair of 

 'saws' which are not unlike the 'stings' of Bees, etc., but are broader, 

 and toothed for a greater length, and are made to slide along a firm piece 

 that supports each blade, like the 'back' of a carpenter's 'tenon-saw;' 

 they are worked alternately (one being protruded while the other is drawn 

 back) with great rapidity; but, when not in use, they lie in a fissure be- 

 neath a sort of arch formed by the terminal segment of the body. When 

 a slit has been made by the working of the saws, they are withdrawn into 

 this sheath; the ovipositor is then protruded from the end of the abdo- 



