LIBRARY

26
COLLEGE ENTOMOLOGY
vidual cells are derived from the vein along the anterior margins of each, and
they are therefore identical with vein names. The common names are somewhat
variable, as shown in the list on page 25:
Abdomen - the third and usually the largest region of the body; composed
of a series of similar segments variable in number, usually 11 in the more primitive
forms. In the higher insects there arc normally nine or 10 vh,ible !'legments,
but they may coalesce, telescope, or be reduced in numbers so that only
three or four may be seen. The abdomen may be rigid in 11l0sL beetles and weevils
but is normally quite elastic, and the terminal segments, at. least, are freely
movable owing to their own resilience and to the thinner ,md lllore flexible texture
of the connective tissues between them. The abdomen is broadly or narrowly
aUached to the thorax. In the latter case the first segment may be closely
fused to the metathorax so as to become more a part of it than of the abdomen.
This fused member, known as the jJropodewn, is common among muny of the
HYMENOPTERA. One or two segments following the propodeum may be
extremely narrow and form a j)edl:cel, while the remainder of the abdomen,
which is more or less globular, triangular, or elongated, is called the gaster.
This condition occurs among hymenopterous parasites, anls, and wasps.
Certain flies have a more or less pedicellate abdomen but lack the propodeum.
On the venter of each of segments II to IX of Machilis, and VIII and IX of
Lepisma in the THYSANURA and I to VII of I aj)yx in APTERA is a pair
of plates, the coxites or coxosterna, each of which bears a single small twosegmented
finger,like stylus, also thought to be rudimentary abdominal legs.
Smaller similar appendages occur on abdominal segments I to III in the
PROTURA. The prolegs of larvre are pseudopods: paired, unsegmented, short
legs which are important organs of locomotion and attachment, varying in
number from two to 10 pairs. In the caterpillars they are furnished with apical
crochets or claws which are absent in the sawflies.
In the COLLEMBOLA there are two organs not found in other insects:
the collophore or ventral tube, a simple or bifid process with eversible sacs which
exude a sticky fluid to help in adhering to objects, and the spring, a small but
powerful two-segmented posterior forked appendage for leaping.
The posterior region of the abdomen is modified in many ways both as to the
character of the terminal segments and of the appendages attached thereto.
Some of these parts are: The cercus (pI. cerci) - a pair of simple or segmented,
antennre-like, tactile organs arising from the lateral posterior margins of seg.
ments X or XI; extremely long in mayflies; modified into a pair of pincers in
earwigs and japygicls. The cercoids - cerci-like appendages arising from segments
IX or X of larvre, are "of a problematical nature" according to Imms
(1937). Median caudal filament - a long cercus-like tactile organ between the
cerci in THYSANURA. Pygidium (pI. pygidia), suranal plate or e/Jiproct - the
last exposed abdominal tergite in beeUes; the terminal unsegmented region in
armored coccids. Propygidium - the tergite (VII) preceding the pygidium in
beetles with short elytra (Dalla Torre).