282 GENERAL ANA TOMY OR HISTOLOG Y OE BLO W-FL Y. 



into fields corresponding with the cells from which it is 

 developed, and this occurs when the latter are united by an 

 intercellular cement. The existence of this cement-substance 

 can be demonstrated by staining with silver nitrate. 



Pore Canals frequently exist in the cuticle. The largest pass 

 into the hollow setae and transmit a process from a trichogenic 

 cell ; smaller, frequently branching, canals pass into papillae, or 

 are lost in the thicker parts of the cuticle. All contain un- 

 differentiated processes of the cells, and may be compared 

 with the dentine tubes of a tooth or the canaliculi of bone. 



The cuticular lining of the stomoda^um and proctodaeum are 

 thin and laminated ; that of the rectum has papilla-like teeth 

 and ridges on its surface in the vicinity of the recto-metenteric 

 valve, and hair-like processes over the rectal papill?e. Cuti- 

 cular membranes also line the tracheae and invest the indi- 

 vidual fat cells. The basement membranes, on which some 

 epithelia rest, must also be regarded as cuticular lamellae. 



4. THE MUSCLES AND NERVES, 

 a. The Muscles. 



The Somatic Muscles of Insects exhibit three varieties, which 

 Weismann [124] distinguished as larval muscle, leg muscle, and 

 wing muscle. Although in their ultimate structure these three 

 forms of tissue are precisely similar, the manner in which the 

 ultimate elements are arranged to form fibres differs, so that 

 except when examined with adequate magnifying power the 

 g'eneral appearance of each is very dissimilar from that of the 

 others. 



In the larva' each muscle consists of one or more large 

 fibres. Each fibre has a distinct muscle - sheath, sarco- 

 lemma, and nuclei are seen at intervals between the sarco- 

 lemma and the muscle-substance. The muscle-substance 

 exhibits distinct transverse striation, and, as Weismann 

 pointed out, the fibres are precisely similar to those of Verte- 



