The Structure and Special Physiology of Insects i 3 
or systems of organs of insects, a special interest attaches to the conditions 
shown by the circulatory and respiratory systems, and by the special sense- 
Fig. 20.—Diagram of lateral interior view of monarch butterfly, Anosia plexippus , show¬ 
ing the internal organs in their natural arrangement, after the removal of the right 
half of the body-wall together with the tracheae and fat body; I to III, segments 
of the thorax; i to 9, segments of the abdomen. Alimentary Canal and Appen¬ 
dages: ph., pharynx; sd. and sgl., salivary duct and gland of the right side; oe., 
oesophagus; f.r., food-reservoir; st., stomach; i., small intestine; c. } colon; r. } rec¬ 
tum; a., anus; m.v., Malpighian tube. Haemal System: h., heart or dorsal vessel; 
ao., aorta; a.c., aortal chamber; Nervous System (dotted in figure): br., brain; 
g., suboesophageal ganglion; l.g., compound thoracic ganglia; ag. v ag. 4 , first and 
fourth abdominal ganglia. Female Reproductive Organs: cp., copulatory pouch; 
v., vagina; o., oviduct, and 00., its external opening; r.ov., base of the right ovarian 
tubes turned down to expose the underlying organs; l.ov., left ovarian tubes in posi¬ 
tion, and ov.c., their termination and four cords; sp., spermatheca; a.gl. v part 
of the single accessory gland; a.gl. 2 , one of the paired accessory glands; only the 
base of its mate is shown. Head: a., antenna; mx., proboscis; p., labial palpus. 
(After Burgess; three times natural size.) 
organs and their manner of functioning. The muscular system varies from the 
simple worm-like arrangement of segmentally disposed longitudinal and 
ring muscles possessed by the caterpillars, grubs, and other worm-like larvae, 
to the complicated system of such 
specialized and active forms as the 
honey-bee and house-fly. Lyonnet 
describes about two thousand dis¬ 
tinct muscles in the caterpillar of 
the goat-moth. Insect muscles are 
similar, in their finer structure, to 
those of other animals, most of Fig. 21.—Bit of muscle of a biting bird-louse, 
them being composed of finely ^urymetopus taurus. (Greatly magnified.) 
cross-striated fibers (Figs. 21 and 22) held together in larger or smaller 
masses and attaching to the rugosities of the inner surface of the exo¬ 
skeleton. The muscle substance, when fresh, is peculiarly transparent 
and delicate-looking, but it has great contractile power. 
The alimentary canal (Figs. 23-27), like that of other animals, is a tube 
but little longer than the body in flesh-eating forms, and much longer in 
plant-feeders; it runs, more or less curving and coiled, through the body 
from mouth to anal opening, which lies in the last segment of the abdomen. 
