Survey of midgut enzymes and midgut pH in insects of different orders

A. B. Dias & W. R. Terra

Dep. of Biochemistry, Univ. of São Paulo, P. O. Box 26077, São Paulo, SP 05513-970, Brazil

The midgut is an important interface between the insects and the environment. Transgenic plants have been produced that affect the digestive functions of the target insects. The development of new transgenic plants needs more data on insect digestive physiology. Considering the great variety of insects, it is necessary the development of generalisations that allow the elaboration of hypotheses relative to an insect for which the there are no data from those yet known. This can be done by studying the digestive system characteristics of insects of different orders followed by a cladistic analysis of the results. With this aim, Periplaneta americana (Dictyoptera: Blattodea) adults, Migdolus fryanus (Coleoptera: Cerambycidae), Sphenophorus levis (Coleoptera: Curculionidae), Corydalus sp. (Megaloptera) and Themos malaisei (Hymenoptera: Symphyta: Argidae) larvae were studied. In M. fryanus and S. levis the midgut is acid in the anterior two thirds (pH~5.0) and more alkaline (pH 6.5~7.5) in the posterior third. In Corydalus sp., the crop and midgut displays neutral pH (pH~7.2), whereas the hindgut is alkaline (pH~8.5). The midgut of T. malaisei is very alkaline: about 9.6 in the anterior two thirds and about 8.5 in its posterior third. P. americana displays an anterior acid midgut (pH~5.7) and a slightly acid posterior midgut (pH~6.1). Amylase, maltase and aminopeptidase were assayed in the midgut of the four insect larvae and adults of P. americana. Based on their activities, amylase and maltase are probably secretory enzymes and aminopeptidase is at least in part membrane bound. Amylase pH optima is acid in M. fryanus, S. levis and Corydalus sp. larvae and P americana adults (pH 5 – 6.5) and it is alkaline in T. malaisei larvae (pH~9.0). Maltase pH optima are similar to those observed for amylase: acid (pH4~5) in M. fryanus, S. levis and Corydalus sp. larvae and alkaline in T. malaisei larvae (pH 7.0). Aminopeptidase has a pH optimum of about 6.8 in M. fryanus larvae, 7.3 in Corydalus sp. larvae and 8.3 in S. levis and T. malaisei larvae. These results are in agreement with our proposal on the evolution of insect digestive systems, although data on T. malaisei suggest that Hymenoptera ancestors may differ from Coleoptera ancestors more than previously assumed.

Index terms: Periplaneta americana, Migdolus fryanus, Sphenophorus levis, Corydalus sp., Themos malaisei.

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