LIFE HISTORY AND FUNCTIONAL MORPHOLOGY OF THE ALIMENTARY CANAL OF Selasia unicolor (COLEOPTERA: DRILIDAE)

Abstract

Selasia unicolor is an important pest of snail feeding exclusively on the flesh of land snails, hence the name “Snail predator”. The adults exhibit sexual dimorphism, having small beetle-like winged males and large apterous larviform females. The larval stage, which is the feeding stage, passes through six (6) larval instar stages before emerging as adults. The major difference across the instar stages is the size; each larval instar is different from the other in length. This study was carried out to determine the life history, the description and behaviour of S. unicolor as well as the anatomy and histology of the gut of S. unicolor larvae. Secondly, the optimal conditions for proteolytic enzymes in the gut of the snail predator were also determined using appropriate biochemical techniques. Larvae of S. unicolor were collected from the field and were reared and studied from egg to adult stage under natural conditions. Histological sections of the gut were performed while the activities of protease were determined in the gut homogenates. The first larval instar is the smallest with an average length of 4.3mm±0.011 while the largest is the sixth larval instar with average length of 40.7mm±0.046. The major distinguishing feature of the first instar is the presence of twelve (12) abdominal segment compared to the eleven (11) present in other larval instar stages. The first three larval instars spend an average of 23 days each in the shell while the last three stages spend 32 days each before emerging, with a total of 195 days (6 months) from eggs to adult. Larvae attack snails by first paralyzing them with certain chemicals released from the mouth, pushing them to safe area before entering into the shell head first through the aperture to feed on the snail’s flesh. The percentage attack and emergence increases with size of larva. The size of the adults that emerge is based on the size of the last (sixth) instar larvae; smaller larvae emerge into male and vice versa. After mating, females can lay between 250 to 950 eggs depending on size. Females shrink and die about an hour after laying eggs. Histological sectioning of the gut revealed that the digestive tract of the larvae is clearly divided into three regions; foregut, midgut and hindgut. The midgut is the longest and largest region for all instar stages while the hindgut is the shortest. Just like most protein feeding insects, the gut is short but about a few millimetres longer than the body length. The intestinal tract of S. unicolor comprised of buccal cavity, oesophagus, crop, midgut, ileum, rectum and anus. Histology of the insect’s gut revealed the columnar epithelium of the midgut which is responsible for digestion of food and absorption of nutrients. The hindgut on the other hand is responsible for the reabsorption of water due to the presence of rectal pad. The proteolytic activity in the gut extract of S. unicolor larvae was at its optimum at 40oC, pH 7 and 2.0% substrate (casein) concentration incubated at 60 minutes