DIGESTION 47 



are replaced by the growth of new cells from special cell nests or nidi. 

 In the merocrine type, the cells do not break down. They may dis- 

 charge their secretion, usually through a striated border, without 

 any visible changes in the cell (save what may be detected by refined 

 cytological methods) and that is probably the most common state of 

 affairs. But a special type of merocrine secretion, known as vesicular 

 secretion, is described, in which discrete droplets of fluid are elimi- 

 nated through the striated border. Unfortunately, identical droplets 

 are only too easily produced artificially during fixation, and appear 

 in abundance where during life they cannot be seen at all. But this 

 does not, of course, exclude the possibility that in some cases vesicu- 

 lar secretion may actually take place. Sometimes the cells of the gut 

 contain conspicuous vacuoles, and these are often taken as evidence 

 of secretory activity. But it must not be forgotten that such vacuoles 

 may equally well arise in the course of absorption. In Aphides, fed 

 after a period of fasting, successive waves of secretory activity appear 

 to pass down the stomach. 



The reaction of the gut 



As was discovered long ago by Plateau (1874), the intestinal contents 

 of insects are not as a rule strongly acid or alkaline. The saliva in 

 those forms in which it has been examined is usually neutral ; the gut 

 contents usually slightly acid, having a pH between 6 and 7. The 

 contents of the gut tend, on the whole, to be more alkaline in phyto- 

 phagous insects (pH 8-4-9-0 in some herbivorous Coleoptera, pH 

 8-4-10-3 in the silkworm) and more acid in carnivores; but there are 

 many exceptions. Well-marked acidity (pU 4-8-5-2) may develop 

 in the crop of the cockroach, after a meal of carbohydrates, as the 

 result of acid fermentation by micro-organisms; and sometimes the 

 insect itself is responsible for a quite strongly acid secretion; as in 

 a well-defined region in the intestine of the blowfly maggot, which 

 has a pH of 3-0-3-5, the acidity being due probably to phosphoric 

 acid. 



Digestive enzymes 



The digestive enzymes of insects are, in general, such as might be 

 predicted from the nature of their food. Omnivorous forms, like the 



