Specific trypsin activity in the midguts of Lu. longipalpis. Sand flies were fed with rabbit blood alone or infected with Le. mexicana and trypsin activity (μM/min/mg protein) assayed at 24, 48 and 72 h after the blood feed/infection. Bars represent mean values and standard errors of combined samples derived from 2 independent experiments. Numbers of flies assayed are as indicated by N values. * represents statistical significance at p = 0.002 and ** represents statistical difference at p = 0.015 (U Mann-Whitney).