338 INSECTA. 



which originally open, not into the stomodaeum, but on the surface of 

 the body. They may therefore be regarded as integumental glands, 

 the apertures of which have been drawn into the buccal cavity (?). 

 In the Trichoptera and Lepidoptera, an anterior pair of these glands 

 develops in the anterior and inner angle of the mandibular rudiment 

 (Hatschek, No. 36 ; Patten, No. 65). A second pair which here, 

 as in the Hymenopteran larva, is transformed into spinning glands, 

 belongs to the segment of the second maxillae (Fig. 143 A, sp, 

 p. 286). Carriere, however, reckons it as belonging to the first 

 thoracic segment. When the second maxillae fuse to form the lower 

 lip, the apertures of the paired invaginations are approximated, and a 

 short unpaired efferent duct forms, opening into the buccal cavity 

 (Butschli, No. 11, etc.). 



We should be predisposed to homologise the salivary gland of the Insecta 

 with the glands which open into the mouth in the Myriopoda. This view is 

 opposed by the consideration that the latter, as transformed nephridia, are said 

 to arise from the mesoderm (p. 251), while the salivary glands of the Insecta 

 are purely ectodermal structures. We must therefore leave the question of the 

 homology of these organs and of their relation to similar glands in PeHpdfacs to 

 be decided by further research. 



The Malpighian vessels develop as paired outgrowths of the 

 proctodaeum, which from the very first have a lumen. They are 

 thus ectodermal structures. Two or three pairs usually make their 

 appearance (Lepidoptera, Phryganeidae, Hydrophilus). In those 

 forms which, at a later stage, have a larger number of these vessels, 

 these secondary tubules develop as diverticula of the primary ones 

 (Gryllotalpa, Rathke). 



The Malpighian vessels usually appear only after the development of the 

 proctodaeal invagination, as diverticula of this latter, but in the Hymenoptera 

 (Apis and ChaHcodoma), they form before the proctodaeum develops, as invagina- 

 tions of the ectoderm, and consequently at first open on the surface of the 

 germ-band. They then somewhat resemble in appearance tracheal invaginations, 

 and this perhaps led to their being homologised with the latter, a view which 

 we are unable to share, and which Carriere also (No. 13) did not adopt. 

 Only later do they shift with the developing proctodaeal invagination into the 

 interior of the embryo. 



G. Heart. 



The earliest recognisable rudiment of the dorsal vessel or heart in 

 the Insecta appears as a longitudinal strand of cells (cardioblasts) 

 running along the upper and external border of the dorsal sub- 

 division of the primitive segments (Fig. 170, li, p. 344, and Fig. 

 171, h). During the continuous circumcrescence of the yolk by 



