268 Comparative Physiology 



with very well developed Golgi apparatus and rich in mito- 

 chondria, a second region with an expanded lumen and rather 

 flattened epithelia in which the secretion is stored, and the third 

 in which sericin (the second silk protein) is probably secreted and 

 applied to the fibroin threads. In this latter region the epithelium 

 has a striated appearance not unlike that of the striated duct cells 

 of the mammalian salivary glands. It is thought that the glands of 

 Lyonnet may provide a lubricant for the silk press. The silk organ 

 is comparatively large and would seem to be an admirable object 

 for the study of secretory problems at the cellular level. 



In a large number of insects digestive enzymes are found in the 

 salivary glands indicating the role of the glands in digestion. For 

 instance, Wigglesworth (1927, 1956) records the presence of an 

 active amylase in cockroaches (Blatella germanica and Periplaneta 

 americana). Its activity was enhanced by halide ions in a similar 

 way to mammalian amylase but its pH optimum was more acid. 

 Invertase was also present but maltase and lactase were not. 

 Baptist (1942) found amylase in the salivary glands of non-blood- 

 sucking members of the hemiptera and heteroptera. The distribu- 

 tion of digestive enzymes in the salivary glands in various orders of 

 insectivora have been summarized by Wigglesworth (1956) and 

 Roeder (1953). 



In blood-sucking insects such as the tsetse fly (Glossina sub- 

 morsitans and Gl. tachinoides) there are no digestive enzymes in the 

 saliva which, however, has instead anticoagulant activity (Wiggles- 

 worth, 1929). In these species the blood is stored undigested in the 

 crop. Lester and Lloyd (1928) showed that digestion in glossina 

 was not impaired after removal of the salivary glands. Other blood- 

 sucking insects (i.e. Cimex and Stomomys) also secrete an anti- 

 coagulant in the saliva. An interesting specialized digestive func- 

 tion of the salivary glands occurs in the larva of Chironomus 

 plumosus L. (Walshe, 1947). This larva first of all makes itself a 

 tubular burrow and then maintaining itself in position by its 

 posterior prolegs performs a number of rotating movements with 

 the anterior part of the body. In this manoeuvre as the head des- 

 cribes complete circles of alternating directions around the circum- 

 ference of the burrow the anterior proleg draws out strands of 

 saliva by rapid approach to and withdrawal from the mouth parts. 

 In this way a loose saucer-shaped sheet of salivary threads is 

 formed. The larva then withdraws a few millimetres down the 



