INSECTS. 111 
its prey ; the Avgyronetra aquatica lives in air-bubbles under water; 
and the Attus volans of Australia has flaps or wing-like extensions of 
the abdomen, that it elevates or depresses during leaps from plant to 
plant. The egg-sacs (Fig. 132, a) are sometimes attached to the webs, 
carried about by the mother, or affixed to stalks (Fig. 132, 4), and re- 
semble small plants. 
VaALUE.—Spiders are useful in destroying other noxious insects. 
The silk is of value to opticians as cross-lines in optical mstruments, 
also as a mechanical styptic. The silk of certain spiders has been 
woven. One of the kings of France possessed a coat made of this silk. 
In Bermuda the silk of MepAi/a has been used as sewing-silk. 
Works on Spiders for further reference. 
“Structure and Habits of Spiders,” J. H. Emerton ; “ Harvesting 
Ants and Trap-door Spiders,” J. T. Moggridge ; ‘‘ The Triangle Spi- 
der,” B. G. Wilder, “ Popular Science Monthly,” 1875 ; ‘‘ Practical 
Use of Spider-Silk,” B. G. Wilder, the “Galaxy,” July, 1869; ‘‘ Pedi- 
palpi of North America,” H. C. Wood, Jr., ‘‘ Journal of Philadelphia 
Academy of Natural Science,” vol. v ; ‘‘ Mites, Ticks, and other Aca- 
ri,” “Popular Science Monthly,” vol. xiv; “Termayer's Researches 
on Spiders’ Silk,” ‘ Proceedings of Essex Institute,” vol. v; “ Pha- 
langeree of the United States,” H.C. Wood, Jr., “ Proceedings of Essex 
Institute,” vol. vi; ‘“ Harper’s Monthly,” vol. lx, Treat. Packard's 
‘Guide to the Study of Insects.” 
Sub-Class IV.—S1x-LeccEp Insects (Hexapoda). 
General Characteristics The Hexapods have antennz 
and two pairs of jaws or maxille. 
Fic. 133.—Glacier-flea (Desora glacialis). 
Order I. Spring-Tails (Zysanura).—The spring- 
tails are minute wingless forms that possess a forked spring, 
