Collembola

The Collembola are minute arthropods. Together with the Diplura and Protura, they are sometimes grouped as the entognathous Hexapoda. The Collembola and Protura are now considered to be classes within the Arthropoda with a close relationship with firstly, the Insecta and secondly, the Crustacea. Although Collembola are not now considered to be insects (Kristensen 1991), for convenience they are often grouped with them. Molecular studies are now beginning to shed light on the relationships of classes within the Arthropoda but so far have not produced definitive results. However, the monophyly of hexapoda on mophological grounds is accepted by several authors (Cameron et al. 2004; Gullan & Cranston 2000; Luan et al. 2005). For many years Collembola were considered to be primitive insects on the basis of their having, like insects, a pair of antennae inserted anteriorly on the head, a thorax with three pairs of legs, and a segmented abdomen with appendages. However, they differ from insects in several features: they are soft-bodied; the mouthparts are internal and enclosed in extensions of the labrum, labium and plicae orales; wings, malpighian tubules and metamorphosis are absent; the abdomen is always primitively six segmented; tracheae are rarely present; and there is, primitively, a springing organ or furca ventrally, arising from abdominal segment IV. The furca at rest is flexed forward against the body and held in place by a hooked organ, the tenaculum, inserted ventrally on abdominal segment III. Another unique feature of Collembola is the ventral tube on abdominal segment I, which carries a pair of extrusible sacs that are involved in water and salt regulation and, in some species, functions as an adhesive organ. In common with Protura, 'salivary' glands are present in the head. These glands have ducts that connect to the buccal cavity. There are also a pair of nephridia in the head that empty via a duct to a ventral groove which leads to the ventral tube.