Bugs, Cicadas, Aphids, and Scale-insects 221 



to be expanded by becoming suddenly filled with blood, and contracted 

 by a receding of the blood. 



The eggs are laid either under bark or on the surface of leaves or, in 

 the case of certain species which have a sharp little ovipositor, underneath 

 the leaf-epidermis. They hatch in from three to fifteen days, varying with the 

 different species observed, and the young grow and feed for from five to 

 forty days. Then follows the brief, quiet, non-feeding stage, and the insect 

 becomes mature. Probably several generations appear in a year. The 

 winter is passed in either larval, pupal, or adult stage, under bark, in dry, 

 hollow plant-stems, in lichens or moss, or on the ground under fallen leaves. 

 A curious variation in the adults of many species has been noted in reference 

 to the wings; adult individuals of a single species may have either fully 

 developed wings, very short functionless wings, or even none at all; both 

 sexes may be winged, or one winged and the other not; one or both sexes 

 may be short-winged or both be wingless. There seems to exist a condi- 

 tion somewhat like that in the plant-lice (Aphididae) , wings being developed 

 in accordance with special needs or influences, as scarcity of food, time of 

 the year, etc. 



Another peculiarity of the adults is the rarity, and even, apparently, 

 the total lack of males in some species. Parthenogenetic development (the 

 production of young from unfertilized eggs) is very common throughout 

 the order. 



While the food of those thrips most easily found by the beginning student 

 is the sap taken from flower parts, most of the sap-drinking species get their 

 supply from the leaves of various plants, and when these plants happen to 

 be cultivated ones of field or garden, and the thrips are abundant, these 

 tiny insects get the ugly name of "pests." Five species in particular 

 are recognized by economic entomologists as pests, viz., the onion-thrips 

 (Thrips tabaci], the wheat-thrips (Euthrips tritici), the grass-thrips (An- 

 aphothrips striatus), the orange-thrips (Euthrips citri), and the pear- thrips 

 (Euthrips pyri). The first of these is about -^ inch long, about one- 

 fourth as wide as long, and of a uniformly light-yellowish to brownish- 

 yellow color. It feeds on many different cultivated plants, as apple, aster, 

 blue grass, melons, clover, tobacco, tomato, cauliflower, etc., etc., but its 

 chief injuries seem to be to onions and cabbages. It occurs all over Europe, 

 England, and the United States, and is probably the most injurious species 

 in the order. The wheat-thrips, also but ^ inch long, brownish yellow 

 with orange-tinged thorax, attacks many plants besides wheat, and is very 

 fond of puncturing the pistils and stamens of strawberry-flowers, thus often 

 preventing fertilization and consequent development of fruit. The life- 

 cycle of this species is very short, requiring only twelve days. Eggs depos- 

 ited in the tissues of infested plants hatch in three days, the larvae are full- 



