68 



ENTOMOLOGY 



substance and the remainder consists of protoplasm and gum, with traces 

 of wax, pigment, fat and resin. 



A transverse or radial section of a silk gland shows a layer of glandu- 

 lar epithelial cells, with the usual intima and basement membrane (Fig. 

 105); the cells are remarkably large and their nuclei are often branched; 

 the intima is distinctly striated, from the presence of pore-canals. The 

 glands arise as evaginations of the pharynx (ectodermal) and the chi- 

 tinous intima of each gland is cast at each moult, along with the general 

 integument. 



The silk glands of Trichoptera are essentially like those of Lepidop- 

 tera, but the glands of Chrysopa, Myrmeleon, Coccinellidae, Chrysomeli- 

 dae and Syrphidae, which open into the rectum, are morphologically quite 

 different from those of Lepidoptera. 



3. MUSCULAR SYSTEM 



The number of muscles possessed by an insect is surprisingly large. 

 A caterpillar, for example, has about two thousand. 



The muscles of the trunk are segmentally arranged most evidently 



abc 



FIG. 106. 



FIG. 107. 



FIG. 108. 



Muscles of cockroach; of ventral, dorsal and lateral walls, respectively, a, alary muscle; 

 abc, abductor of coxa; adc, adductor of coxa; ef, extensor of femur; h, head muscles; Is, 

 longitudinal sternal; It, longitudinal tergal; Ith, lateral thoracic; os, oblique sternal; ot, 

 oblique tergal; ts, tergo-sternal; ts l , first tergo-sternal. After MIALL and DENNY. 



so in the body of a larva or the abdomen of an imago, where the muscu- 

 lature is essentially the same in several successive segments. In the 

 thoracic segments of an imago, however, the musculature is, at first sight, 



