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Plate XXXIII. - SILK SECRETORS OF THE SILKWORM-SPIN
NERETS OF THE SPIDER-PANORPA COMMUNIS-FEMALE AND MALE GLOW-WORMLARVA LIBELLULÆ-BREATHING SPIRACULA-PUPA OF GNAT-STRATYONIS CHAMELEON.
Fig. 1. The organs for forming the silk consist of two long vessels. They unite to form the spinneret (fusulus) through which the larva draws the silken thread employed in fabricating its cocoon. a, a, the silk bags, b, the spinneret.
Fig. 2. The web of spiders is also a kind of silk, remarkable for its lightness and tenuity; it is spun from four or six anal spinnerets, the fluid matter forming the web being secreted in adjacent vessels.
a, b, c, d, the spinnerets.
Fig. 3. Panorpa communis, (Linn.) is an insect frequently seen in meadows during the early part of summer. It is a long-bodied fly, of moderate size, with four transparent wings, elegantly variegated with deep brown spots.
Fig. 4. The female glow-worm.
Fig. 6. The larva of some dragon-flies (@shna and libellula, F.) swim by strongly ejecting water from the anus. By first taking in the water, and then expelling it, they are enabled to swim. This may be seen by putting one of these larva into a plate with water. We find that while the animal moves forward, a currant of water is produced by this pumping in a contrary direction. Sometimes it will raise its tail out of the water, when a stream of water issues from it.
Fig. 7. The spiracula, or breathing pores of insects, are small orifices in the trunk or abdomen, opening into a canal called the trachec; by which the air enters the body, or is expelled from it. In the larvæ or caterpillars, a trachea runs on each side of the body, under the skin, and generally opens externally by nine or ten apertures or spiraculæ ; from these the same number of air-vessels of a silver color pass off to be dispersed through the body. a, a, spiracula; b, b, trachea.
Fig. 8. The pupă of gnats suspend themselves on the surface of the water, by two auriform respiratory organs on the anterior part of the trunk, their abdomen being then folded under the breast ; when disposed to descend, the animal unfolds it, and with sudden strokes which she gives with it and her anal swimmers to the water, she swims from right to left, as well as upwards and downwards, with the greatest ease.
Fig. 9. This is a well known Ay, (stratyonis chamæleon, F.) chameleon fly. In its first state it inhabits the water, and often remains supported by radiated tail, consisting of beautiful feathered hairs or plumes, on the surface, with its head downwards. But when it is disposed to seek the bottom or to descend, the radii of the tail is formed into a concavity including in it an air bubble ; this is its swim bladder, and by the bending of its body from right to left, contracting itself into the form of the letter S, and then extend'ng itself again into a straight line, it moves itself in any direction.