54 THE LARVA OF THE BLOW-FLY. 



opens at the tip of or under the ligula. In many insects they 

 secrete a true digestive fluid, with proteolytic or amylolytic 

 ferments; in others they are silk glands (sericteria). I shall 

 term them lingual glands, a term which is equally appropriate 

 whatever their function. 



The other salivary glands may be distinguished as accessory 

 lingual, labial, oral, or pharyngeal glands, according to the 

 position of the orifices of their ducts. 



Moseley regards the lingual glands as modified cutaneous 

 glands, homologous with the tracheae. They are certainly 

 developed as involutions of the epiblast at the root of the 

 second pair of maxillae, but differ in their mode of development 

 from the tracheae. The only reason for considering that they 

 are homologous with the latter is the spiral structure of the 

 intima of their duct. 



The Gastric Glands, or gastric caeca (PI. I., Fig. 3, eg) are 

 tubular glands arising from the walls of the chyle stomach. 

 They are frequently very numerous, but as often entirely 

 wanting. 



The Malpighian Vessels are simple or branched tubules, con- 

 taining large cells, which give them a moniliform appearance. 

 In some insects they are very numerous. In most, if not in all, 

 Diptera there are two, each dividing into two tubes (PL I.. 

 Fig. i, inp). I know no insect in which they are entirely 

 absent. Malpighi supposed these tubes to be hepatic : 

 Leydig and most modern writers consider them renal. 

 This view I shall show hereafter is not consistent with facts ; 

 and, in spite of the weight of authority, I strongly incline to 

 the theory of Malpighi. 



The Rectal Glands are situated in the proctodaeum. They are 

 frequently wanting, and apparently exhibit great variations in 

 different insects. In the larva of the fly they are entirely 

 absent. In the perfect insect I think they are undoubtedly 

 concerned in the excretion of a substance allied to, but not 

 identical with, uric acid.* 



* The functions of these and of the other internal organs will be fully dis- 

 cussed in the physiological section of this work. 



