The following bulletins of the Bureau of Entomology may be obtained from the same source at the prices affixed: Bulletin No. 25 (old series), Destructive Locusts, 15c.; No. 1 (new series), The Honey Bee, 15c.; No. 3, The San José Scale, 10c.; No. 4, The Principal Household Insects of the U. S., 10c.; No. 11, The Gypsy Moth in America, 5c.; No. 14, The Periodical Cicada, 15c.; No. 15, The Chinch Bug, 10c.; No. 16, The Hessian Fly, 10c.; Nos. 19, 23, and 33, Insects Injurious to Vegetables, 10c. each; No. 25, Notes on Mosquitoes of the U. S., 10c.; No. 42, Some Insects attacking the Stems of Growing Wheat, Rye, Barley, and Oats, 5c.; No. 50, The Cotton Bollworm, 25c.; No. 51, The Mexican Boll Weevil, 25c. Bureau of Plant Industry--Bulletin No. 88, Weevil-resisting Adaptations of the Cotton Plant, 10c. This gives an instructive account of the struggle of a plant for existence against an insect enemy. [Illustration: FIG. 186.--Gall fly (enlarged) and oak gall with larva, and one from which a developed insect has escaped.] [Illustration: FIG. 187.--PLAN OF MOUTH PARTS OF THE INSECT ORDERS. _A_, straight wings, nerve wings, false nerve wings; _B_, joined wings; _C_, scaly wings; _D_, half wings; _E_, two wings. _ol_, upper lip; _ok_, biting jaws; _uk_, holding jaws; _ul_, under lip; _kt_, jaw fingers; _lt_, lip fingers.] [Illustration] [Illustration: Pearl divers.] CHAPTER IX MOLLUSKS THE FRESH-WATER MUSSEL SUGGESTIONS.--The mussel is usually easy to procure from streams and lakes by raking or dredging. In cities the hard-shelled clam, or quahog, is for sale at the markets, and the following descriptions apply to the anodon, unio, or quahog, with slight changes in regard to the siphons. Mussels can be kept alive for a long time in a tub with sand in the bottom. Pairs of shells should be at hand for study. =External Features.=--The shell is an elongated oval, broader and blunter at one end (Fig. 188). Why does the animal close its shell? Does it open the shell? Why? Does it thrust the foot forward and pull up to it, or thrust the foot back and push? (Mussels and clams have no bones.) Does it go with the blunt or the more tapering end of the shell forward? (Fig. 188.) Can a mussel swim? Why, or why not? Lay the shells, fitted together, in your hand with _the hinge side away from you and the blunt end to the left_ (Fig. 188). Is the right or the left shell uppermost? Which is the top, or dorsal, side? Which is the front, or anterior, end? Is the straight edge at the top or the bottom? Our word “valve” is derived from a word meaning shell, because the Romans used shells for valves in pumps. Is the mussel a univalve or a bivalve? Which kind is the oyster? The snail?