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- 



br 





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 g, segments of the abdomen. Alimentary Canal and Appen- 

 dages: ph., pharynx; sd. and sgl., salivary duct and gland of the right side; oe., 

 oesophagus; j.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.,, ag.„ first and 

 fourth abdominal ganglia. Female Reproductive Organs: cp., copulatory pouch; 

 v., vagina; c, oviduct, and oo., 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.^, part 

 of the single accessory gland; a.gl.„, 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 Fio. 21. 

 them being composed of finely 

 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. 



-Bit of muscle of a biting bird-louse, 

 Etirymetoptis laiirus. (Greatly magnified.) 



