142 INSECTS CHAP. IV 



genesis the young are produced alive instead of in the form of 

 eggs. A very rare kind of parthenogenesis, called paedogenesis, 

 has been found to exist in two or three species of Diptera, 

 young being produced by the immature Insect, either larva or 

 pupa. 



Glands. 



Insects are provided with a variety of glands, some of 

 which we have alluded to in describing the alimentary canal 

 and the organs of sex ; but in addition to these there are others 

 in connexion with the outer integument ; they may be either 

 single cells, as described by Miall in Dicranota larva,-^ or groups 

 of cells, isolated in tubes, or pouches. The minute structure 

 of Insect glands has been to some extent described by Leydig ; ^ 

 they appear to be essentially of a simple nature, but their special 

 functions are very problematic, it being difficult to obtain 

 sufficient of their products for satisfactory examination. 



^ Tr. Ent Soc. London, 1893, p. 241. 2 j^^^h. Anat. Phys. 1855 and 1859. 



