6 MISCELLANEOUS STUDIES 



to the primary sensorium on the terminal segment. Secondary sen- 

 soria are usually present in the alate forms, but oftentimes absent 

 in the apterae of certain species. When present they are always on 

 the third segment, but in antennae consisting of five or six segments, 

 they may be present upon the fourth, fifth, and even sixth segments. 

 In the Pemphiginae they are arch-like or half rings, or form complete 

 rings about the segments. In the Aphidinae they are circular, oval, 

 or transversely linear, but are never rings or half rings. The shape 

 and number vary considerably, and are of specific importance. The 

 number may vary from as few as three or four (Myzocallis maureri 

 Swain), to as many as forty to fifty on the third segment, and many 

 also on the fourth and fifth (Myzus braggii Gillette). Unfortunately 

 these highly important characters were overlooked or not taken into 

 consideration by the earlier workers. The beak is four-jointed and 

 seems to arise from between the fore legs. It is always present (except 

 in the sexes of certain of the Phylloxerinae), but is seldom of specific 

 importance (except to distinguish Aphis bakeri Cowen from Aphis 

 senecio Swain, and in certain of the Lachini). It may be very short, 

 as in Aphis bakeri Cowen, where it reaches only slightly beyond the 

 first coxa, or it may be very long as in Stomaphis, where it is from 

 one and one-half to two times as long as the body. In leaf-feeding 

 species it is usually short, while in bark-feeding forms it is longer. 

 This is naturally necessary, for those that live on thick bark must have 

 a longer beak in order to reach through to the plant juices. 



The thorax consists of three divisions, the last two of which are 

 usually more or less fused together, and considered as one ; the two 

 divisions being called, in this paper, the prothorax and the thorax. 

 On the lateral margins of the prothorax there is sometimes a pair of 

 small tubercles. These are not present in all species, however, and 

 they differ considerably in size in the various species. There are 

 three pairs of fairly long and slender legs (except in Phylloxerinae. 

 where the legs are greatly atrophied, approaching those of the 

 Coccinae in size). Typically the legs consist of four joints, the 

 coxa, the femora, the tibia, and the tarsus. In some genera the tarsi 

 may be atrophied (Atarsos, Mastapoda}. The comparative lengths 

 of the first and second segment of the tarsi are sometimes of generic 

 importance (Lachnini), and the comparative lengths of the hind tarsi 

 and the cornicles are oftentimes of specific importance (Aphis, Ptero- 

 comma). A small empodial hair is found between the claws in the 

 Aphidinae. In the Callipterina it is leaf-shaped or spatula-like. In 



