muscular power feeble, and allowing but comparatively little activity. 
This condition of the external skeleton renders these insects very sus¬ 
ceptible to injury. The body is soft and yielding and usually ovate, 
or oval in form; and when contracted by the partial losses of their 
juices, the skin is thrown into more or less marked folds or wrinkles. 
It is divided into rings or segments, but the number does not appear 
to be constant; as in some species certain rings are hidden or wanting. 
The division into head, thorax and abdomen is manifest, and is very 
distinctly marked between the two former (head and thorax); but in 
many wingless species, especially some of the subterranean ones, the 
thorax and abdomen are only distinguished by noting the number of 
the segments or those that bear the legs. 
The various appendages to these divisions will be noticed as each 
division is considered. 
The Head. 
The head in plant-lice is sufficiently distinct to be recognized by 
the most unscientific observer. It is usually nearly quite as broad as 
the front of the thorax. Seen from above it is transverse, the width 
exceeding the length ; the front fiat, slightly concave, or with the mid¬ 
dle portion slightly advanced in a triangular form. Seen from the 
front it is triangular, with the base-line above extending from one 
eye to the other, and the apex below terminating in the beak. Seen 
from the side it also presents usually a triangular outline approaching 
a right angle triangle, the top and back forming the base and per¬ 
pendicular, and the front the hypothenuse, which is sloped strongly 
under towards the breast. In some genera, especially Siphonophora , 
seen from the side, it reminds one very strongly of the head of a 
goat. Tn fact so marked is this that it often assists in determining 
the generic relation of a specimen which is injured or imperfect. To 
it appertains the following parts and appendages: 
Beak .—Whi oh is sometimes designated by the terms Rostrum , Pro- 
hoscis , cfic., is the prolonged and jointed mouth which arises from the 
back part of the underside of the head, and when at rest is usually 
turned under and pressed up against the breast, but is often seen di¬ 
rected obliquely downwards and backwards apparently from a point 
between the fore-legs. 
It is divided into three joints, of which the basal or first is usually 
much the longest. It is in fact an extension or prolongation of the 
under lip ; the first joint being deeply channelled above for the pur¬ 
pose of receiving and protecting the slender, hair-like setae or piercers, 
which are the weapons these insects use to pierce the leaf or bark on 
which they feed ; the second or middle joint is perforated instead of 
being channelled, thus securing the slender piercers in their position ; 
the third joint, is channelled above, and pointed at the tip. With a 
magnifier, the slender setae (of which there are three) may often be 
seen thrust out considerably beyond the point of the beak by the 
living insect, darting out and in like the tongue of a serpent. These 
organs, which represent the jaws and mandibles of other insects, are 
the real instruments with which the plant-lice first pierce the plants, 
which is not done by the beak as is generally supposed. Its length, 
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