It is of interest to consider the function of these mouth-parts and what type of feeding is possible in this collembolan. The labium is divided into separate halves for almost the entire length of the cone and this division, together with the lateral extensions of the mouth allows the distal portion of the cone to open when the mandibles and maxillae may be extended beyond the mouth opening (Figs. 1, 2). As the maxilla is hooked apically it would not appear to act as a piercing stylet. The mandibular shaft is rod-like, roughly circular in cross section rather than a flat shaft and the head is not a flat cutting plate as previously described (Massoud 1967), but is modified to form a cupped, claw shaped structure. The mandibular head therefore cannot be termed a "pars incisiva" as the teeth do not form a cutting edge, and the action of the mandibles is no longer a transverse cutting action, but a protraction and retraction (Wolter 1963).
As previously discussed, Collembola of this type have long been regarded as sucking, or piercing and sucking their food. It was considered that owing to the small size of the apical mouth opening they are not capable of conveying any solid food particles into the mouth cavity. The consistent absence of visible gut contents observed by a number of writers, including Macnamara (1924) and Poole (1959), was page 6
Fig. 3 Ceratrimeria leleupi
Distal portion of cone with mandible and maxillae partly extruded. Lower maxilla shows double structure and mandibles carry bacteria. M, mandible; Mx, maxilla; Lb, labrum (× 2,500).
Fig. 4 Ceratrimeria lelupi
Mandibular head showing claw-like form and containing bacteria (× 5,000).
considered to indicate that they are fluid feeders. The entire mouth cone was said to function as a suction tube attaching itself firmly to the surface of the substrate during feeding.
The mandibles and maxillae of the present species are clearly not sucking structures and with the loss of the molar plate comminution of food particles by the mandibles is not possible. The structure of the mouth opening with thickened folded borders and divided labium bears a strong resemblance to that observed in Brachystomella parvula
(Schaeffer) by Adams and Salmon (1972). In B. parvula
the terminal portion of the cone remains open during feeding and the maxilla heads and superlinguae make rapid movements in and out of the mouth openings. Specimens of the present species were found with the cone open and the mouth-parts extended to varying degrees. As noted above, in forms possessing a buccal cone, the action of the mandibular and maxillary muscles is protraction and retraction. It seems reasonable to
Fig. 5 Ceratrimeria leleupi
Distal end of maxilla, carrying bacteria of branching and coccoid form (× 7,000).
assume therefore, that the action during feeding in this species consists, as in B. parvula
, of an opening of the distal portion of the cone which allows the heads of the mandibles and maxillae to emerge and operate in a protraction-retraction movement conveying food particles in to the mouth. As the mandibles and maxillae are of minute size and lack grinding or chewing plate, the size of food particles or organism ingested is accordingly limited. It has been suggested by Christiansen (1964) that bacteria may form an important part of the collembolan diet and several species were cultured on bacteria as the sole source of food. These however were not visible within the gut after ingestion. In the present study, bacteria of bacillus and coccoid form were observed on the mandibular heads of a number of specimens (Figs. 3
). Both mandibles were found to contain bacteria with very little other material present. In other specimens, although the mouth-parts were often fully extruded, the cupped mandibular heads appeared empty. Bacteria were the only identifiable material found on the mandibles. Similarly, bacteria of coccoid and branched form were found adhering to the maxillae, concentrated particularly at the distal end (Fig. 5
). Bacteria were not observed on any other parts of the body. The consistent lack of visible gut contents in this species and the presence of bacteria on the mandibles and maxillae suggests that small organisms such as bacteria are at least a source of food.
The question then arises as to the function of the buccal cone. If it does not act as a suction tube, does it serve some other function in food selection? It has been noted that in Collembola possessing a buccal cone, the structure and function of the mouth-parts has changed. The molar plate with its grinding action is lacking and the mandibles, if present, are modified to slender, elongated, extremely delicate structures. With this modification there is a consequent reduction in the size of food particles ingested. The narrowing of the wide frontal mouth region to a tapered buccal cone appears to have evolved with this structural change in the mouth-parts and it seems probable that the cone serves to support and protect the delicate protrusible mouth-parts.