COLLEMBOLA 



227 



Fig. 233.— Hind foot of Ach- 

 orutes maturus. (After Fol- 

 som.) 



who studied the development of the mouth-parts of Anurida maritima, 

 that, strictly speaking, the jaws are not "retracted," as is usually 

 stated , but are overgrown by the genas. In an early embryonic stage, 

 a downward projection of the gena ap- 

 pears on each side of the head, and these 

 "mouth -folds" become larger and larger 

 in successive stages until the condition 

 seen in the fully developed insect is 

 reached. 



The development of mouth-folds is 

 not restricted to the Collembola, but 

 occurs also in the Entotrophous Thysan- 

 ura, and to a less marked extent in many 

 of the Pterygota, especially in some 

 Orthoptera, where the gena of each side 

 is prolonged into a small, but distinct, 

 flat fold over the base of the mandible. 



In some of the Poduridas the mouth- 

 parts are fitted for piercing and sucking, 

 the mandibles and maxillae being styliform 

 and projecting in a conspicuous cone. 



In some of the Collembola there is a 

 sense organ situated between the base of the antenna and the ocular 

 field; this is known as the postantennal organ; its presence o^ absence 

 and its form when present afford characters used in the description 

 of these insects. In its simplest form it is a claviform hyaline tubercle 

 {Sminihurus) . A more complicated type is that of Anurida maritima, 

 which has been figured by Willem ('00). In Figure 232, Pa repre- 

 sents a longitudinal section of this organ. It is a nerve-end-cell, 

 branching from the optic nerve and extending to the surface of the 

 body, where it is covered by a very thin cuticular layer. It is pro- 

 tected by a ring of tubercles {t, t), two of which are shown in the 

 sectional view (A) and eight in the surface view (B). The function 

 of this organ has not been determined; it has been suggested that it 

 is an organ of smell. 



The legs of the Collembola consist each of five segments, which 

 correspond to the five principal divisions of the legs of the higher 

 insects. Willem ('00) considers the two antecoxal pieces as segments 

 of the legs and consequently states that the legs are composed of 

 seven segments The tarsi in most genera bear two claws, an outer, 

 larger one, the unguis, and an inner, smaller one, the unguiadus; these 

 claws are apposable (Fig. 22,^); in some genera the inner claw is 

 wanting. 



One of the most characteristic features of the Collembola is the 

 collophore, or ventral tube, which is situated on the ventral aspect of 

 the first abdominal segment (Fig. 230, co). This organ varies greatly 

 in form in the different genera; in some it is a simple tubercle, di- 

 vided into two halves by a central slit; in others it is enlarged and 

 becomes a jointed tube divided at its free end into two lobes. The 



