226 



AN INTRODUCTION TO ENTOMOLOGY 



of snow, where they appear as minute black specks, which spring 

 away on either side from our feet as we walk; and some species 

 collect in great numbers on the surface of standing water. Sev- 

 eral species are known to be photogenic. 



The body consists of the head, three thoracic segments, and six 

 abdominal segments (Fig. 230). The prothorax is usually small and 

 in several genera is overlapped by the tergum of the mesothorax ; in 

 theSminthuridffi the body-segrnents are more or less fused together. 

 The structure of the abdomen is remarkable, as it consists of only six 

 segments; there is no indication of the manner in which the reduc- 

 tion of the number of segments has taken place. The anus is at the 

 caudal end of the body; the genital opening is on a small papilla 

 on the fifth abdominal segment. 



The antenna consist of from four to six segments, usually of four. 

 They vary greatly in their comparative length; in some genera the 

 last segment or the last two segments are divided into many rings 



orsubsegments(Fig.23o). 

 The eyes of the Col- 

 lembola are commonly 

 described as a group of 

 eight, or fewer, distinct 

 simple eyes on each side 

 of the head. But these 

 so-called simple eyes are 

 not ocelli; they are more 

 or less degenerate omma- 

 tidia, each group being 

 the vestige of a com- 

 pound eye. In Podura 

 aquatica, these eyes, as 

 figured by Willem ('00), 

 are clearly ommatidia of 

 the eucone type (Fig. 

 231). In some other Col- 

 lembola, as in Anurida 

 niaritima (Fig. 232, O), 

 the reduction of the om- 

 matidia has progressed 

 so far that they present 

 the appearance of ocelli; and in still others the eyes are lost entire- 

 ly. Primary ocelli have not been found in the Collembola. 



The mouth-parts are typically mandibulate; the jaws consisting 

 of a pair each of mandibles, paragnatha, and maxillae. The parag- 

 natha of Orchesella cincta were described by Folsom ('99) ; and those of 

 Anurida niaritima by the same writer ('00). These organs were 

 termed the siiperlingiiCB by Folsom. 



One of the most striking characteristics of the Collembola is that 

 the jaws are apparently retracted into the cavity of the head so that 

 only their tips are visible. But it has been shown by Folsom ('00), 



Fig. 232. — A, longitudinal section of an ommatid- 

 ium and of the postantennal organ of Anurida 

 niaritima; B, a surface view of the postantennal 

 organ. (After Willem.) O, ommatidium ; Pa, post- 

 antennal organ; hy, hypodemial cells; N, optic 

 nerve; n, branch of the optic nerve; /, t, tuber- 

 cles surrounding the postantennal organ;,, g, 

 nerve-end-cell of the postantennal organ. (After 

 Willem.) 



