48 INSECT PHYSIOLOGY 



cockroach, have a full complement of enzymes capable of digesting 

 all the common foodstuffs. Insects which take chiefly a protein diet, 

 such as the exclusively blood-sucking species, have little but proteo- 

 lytic enzymes. Where an insect feeds only on nectar, as in the adults 

 of Lepidoptera, only an invertase is present; whereas the phytopha- 

 gous larvae of these same forms have a protease, lipase, amylase, 

 maltase, and invertase. Table I gives some examples in simplified 

 form to illustrate this relationship, the enzymes being given the 

 names that were used in the past. The cockroaches Blaberus and 

 Periplaneta are described in current terms as producing a-glucosidase 

 (acting on sucrose and maltose), (3-glucosidase (cellobiose), a-fructo- 

 furanase (raffinose and sucrose), a-galactosidase (melibiose), and |3- 

 galactosidase (lactose). 



These enzymes are readily demonstrated in extracts from the gut. 

 They are present also in the juice removed from the lumen, showing 

 that at least the greater part of digestion is extracellular. As can be 

 seen from Table I most of the enzymes are secreted in the mid-gut; 

 the salivary glands usually secrete only amylase, and often no en- 

 zymes at all; but we have already referred to the varied enzymes in 

 the salivary glands of some plant-sucking Hemiptera (p. 45). 



The properties of the enzymes of insects are, in general, very like 

 those of the corresponding enzymes from vertebrates. They are simi- 

 larly affected by changes in hydrogen ion concentration, and are 

 stimulated or inhibited by the same factors. The chief proteolytic 

 enzyme seems almost always to be more or less like pancreatic tryp- 

 sin, though it often acts best over a rather more acid range of pH. 

 As in vertebrates the peptidases are of three sorts, 'aminopolypepti- 

 dase' and 'carboxypolypeptidase' characterized by their ability to 

 break off amino-acids from the amino end or the carboxyl end of the 

 polypeptide chain, and 'dipeptidase' which hydrolyses dipeptides. 

 Enzymes of the pepsin type acting in a very acid medium (pH 2-3) 

 are found in the maggots of Calliphora and other flies. Certain special 

 enzymes are present in those insects which feed on particular sub- 

 stances; for instance, the meat-eating maggot (Lucilia) has a collage- 

 nase which attacks raw connective tissue in alkaline medium; and a 

 few of the wood-boring beetles produce a cellulase. The clothes moth 

 (Tineold) can digest keratin ; it contains in its gut a powerful reducing 



