Mouthparts of Three Collembolan Species: Comparative Morphology

Abstract

The mouthparts (labrum, mandibles, and maxillae) of three litter-dwelling collembolan species-Orchesella hexfasciata (Entomobryidae), Isotoma notabilis (Isotomidae), and Sminthurinus henshawi (Sminthuridae)-are described, using both light and scanning electron microscopy. The right molar plate of all three species bears a projection that fits into a corresponding depression on the left molar plate, a previously unnoticed but possibly common and important feature. Based on the free-standing nature of molar plate rods, we suggest a brushing rather than a grinding function. The mouthparts of these species are structurally very similar, but differ in size. Size of labrum and mandibles probably dictates the size of food particles ingested. The rake-like structure of lamella 1 on the maxillar head of L. notabilis may explain why the gut contents of this species consist largely of colloidal material. Additional key words: labrum, mandible, maxilla, Orchesella, Isotoma, Sminthurinus Collembolans are dominant arthropods in terrestrial detritus-based food webs, both in terms of population density and species richness. They have an integral role in maintaining and shaping microbial activity and community structure, and are important mediators of food web stability (Moore et al. 1988). The functional importance of collembolans in decomposition processes is closely associated with their feeding activities (Seastedt 1984). Some significant contributions on the structure and function of collembolan mouthparts include Folsom (1899, 1900), Hoffman (1905, 1908), Borner (1908, 1909), Denis (1928), Wolter (1963), Manton (1964), Goto (1972), and Chaudonneret (1973). Although the link between mouthpart structure and feeding habits has long been recognized (MacNamara 1924; Fjellberg 1985), no attempt has been made to investigate the relationship between detailed mouthpart morphology and food selection. During an investigation of food partitioning in litter-dwelling collembolans (Chen et al. 1996), we studied the morphology of the labrum, mandibles, and maxillae of three collembolan species common in deciduous forest, using both light and scanning electron microscopy. Collembolans possess an entognathous mouth, mandibles and maxillae being enclosed by the head capsule. Two general mouth types are recognized, depending on whether mandibular molar plates are present (chewing type) or absent (sucking type). Collembolans a Author for correspondence. E-mail: 23118MGR@msu.edu with mandibular molar plates use their mouthparts (mainly mandibles and maxillae) to chew, grind, and/or tear food material before ingesting it. The tips of the mandibles can scratch the food surface, and mandibles and maxillae can also reach out, grasp, and carry food back into the mouth. Movement of mandibles and maxillae is restricted by surrounding structures such as labrum, labium, and the lateral margins of the gnathal pouch (Manton 1964). Shape and size of mandibles and labrum may restrict type and particle size of ingested food (Fjellberg 1985). Although constructed according to the same general model, the feeding apparatus varies among species even in closely related groups (Fjellberg 1984).