220 NATURAL HISTORY OF ARTHROPODS. 



are remarkable for being attached to the mesothorax, and never to the metathorax, 

 as in other Homoptera. Here, as in the Coccidae, the mesothorax is much the largest 



segment ; the prothorax is usually little more than 

 a ring closely compacted against the former, while 

 the metathorax is a very small segment rendered 

 inconspicuous by the base of the abdomen. Accord- 

 ingly, in the winged individuals, the hind-legs are 

 set pretty far back, as if they arose from beneath 

 the abdomen, but in the unwinged the legs appear 

 closer together ; the hind ones being often con- 

 spicuous for their length and thickness. 



On each side of the head a prominent round 

 eye is placed, against which may be observed a 

 FIG 299. Aphis tubercle or supplementary compound eye, and 



besides these there are three, rarely six, ocelli 



variously situated on the vertex between the antennae. Most species which live 

 beneath the surface of the ground have neither eyes nor ocelli. The antennae 

 are usually long, in a few very short, slender filiform, thicker at base, and composed 

 of a variable number of joints, never less than three nor more than seven. The 

 rostrum is a slender, three-jointed organ, of variable length, in some genera such 

 as Lachnus and Schizoneitra, it is longer than the abdomen, and projects behind like 

 a tail. 



Typically there should be eleven rings to the abdomen, biTt only nine are conspicu* 

 ous ; and the stigmata are placed on the underside as usual, but difficult to detect. 

 These latter are of very simple structure, have either a circular or oval orifice, consist 

 of ten on each side, and are kept open by a horny ring. On the upper side of the 

 sixth abdominal segment there is usually a pair of cornicles or slender tubes, often ol 

 considerable length, composing the sheaths of excretory ducts, which pour forth a 

 sweet liquor at the pleasure of the insect. 



In some genera, such as Lachnus and its allies, these organs are merely perforated 

 tubercles, while in Phylloxera, and some other low forms, they are entirely wanting. 

 Attached to the end of the ninth abdominal segment there is usually a little slender 

 process called the ca^lda, or tail, which, by its numerous modifications, affords excellent 

 distinctive characters for grouping the species. 



Unlike the members of the preceding family, the plant-lice are active in all their 

 states, and in general present a similarity of appearance, which even in the unwinged 

 and young forms would serve to distinguish them from all other insects. This agree- 

 ment is most noticeable in their generally flask-shaped bodies, crowned by a truncated 

 head and puffed face, the latter feature recalling that of some members of the Pseudo- 

 neuropterous family Psocidre. When quite young they are often narrower, and of a 

 more or less quadrangular form ; while the winged females are less broad and stout 

 than the unwinged ones. 



j 



One great feature of their economy has made them the wonder of all observers from 

 the time of Reaumur, who first gave an extended account of their history, to the pres- 

 ent. But it is to the patient and laborious investigations of Bonnet that we are in- 

 debted for the discovery that these little creatures are capable of indefinite propagation 

 without the intervention of the male. This well-known observer studied individuals 

 of several species from hour to hour for a period of five months, and recorded every 



