338 
CHAPTER XIX. 
Plate XXXIII. — silk secretors or the silkworm—spin* 
NERETS OF THE SPIDER-PANORPA COMMUNIS-FEMALE AND 
MALE GLOW-WORM-LARVA LIBELLULiE-BREATHING SPIRACU- 
L.2E-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, 6, 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. 5. The male of the same insect. 
Fig. 6. The larva of some dragon-flies ( ceshna 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 pump¬ 
ing 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 ori¬ 
fices in the trunk or abdomen, opening into a canal called the trachece; 
by which the air enters the body, or is expelled from it. In the larvae or 
caterpillars, a trachea runs on each side of the body, under the skin, and 
generally opens externally by nine or ten apertures or spiraculae ; from 
these the same number of air-vessels of a silver color pass off to be dis¬ 
persed through the body, a, a, spiracula ; b , 6, trachea. 
Fig. 8. The pupae 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 de¬ 
scend, 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 easei 
Fig. 9. This is a well known fly, ( stratyonis chamceleon , F.) cha¬ 
meleon fly. In its first state it inhabits the water, and often remains 
supported by its radiated tail, consisting of beautiful feathered hairs or 
plumes, on the surface, with its head downwards. But when it is dis¬ 
posed 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 extending itself again into a straight line, it 
moves itself in any direction. 
