146 EVENINGS AT THE MICROSCOPE 



destitute of limbs; and then on the sixth we begin to find 

 another series, which goes on regularly, a pair on each 

 segment, to the eleventh and final one, with the single 

 exception of the tenth segment, which is again deprived 

 of limbs. 



But these organs are of a very peculiar character. 

 They have no representatives in the mature insect, but 

 disappear with the larva state, and they are not consid- 

 ered limbs proper at all, but mere accessory developments 

 of the skin to serve a special purpose. They are some- 

 times called claspers, sometimes false-legs, but more com- 

 monly pro-legs. 



Each consists of a fleshy wart, which is capable to some 

 extent of being turned inside out, like the finger of a 

 glove. Partly around the blunt and truncate extremity are 

 set two rows of minute hooks, occupying the side next the 

 middle line of the caterpillar in a semicircle along the mar- 

 gin. These hooks arch outward as regards the axis of the 

 pro-leg, though the majority of them point toward the 

 medial line of the body. The double row is somewhat 

 interrupted at its middle point; and just there, in each 

 pro-leg, a clear vesicle or fleshy bladder protrudes from 

 the sole, which may perhaps serve as a very delicate organ 

 of touch, or may exude a viscid secretion helpful to prog- 

 ress on smooth bodies. The hooks seem adapted to catch 

 and hold the fine threads of silk, which most caterpillars 

 spin as a carpet for their steps. 



In some cases the circle of hooks is complete, as in this 

 example, which I find in one of the slides of my drawer, 

 marked " Pro-leg of a Caterpillar." It is some large spe- 

 cies, probably a Sphinx, for the hooks are very large, of a 

 clear orange-brown hue, and set in a long oval ring sin- 



