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 

 theSminthuridae the body-segments 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 antennae 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 



or subsegments (Fig . 2 3 o) . 

 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 

 aqtiatica, these eyes, as 

 figured by Willem ('00), 

 Fig. 232.— .4, longitudinal section of an ommatid- are clearly ommatidia of 

 ium and of the postantennal organ of ^Hz<n(/a the eucone type (Fig. 

 maritima; B, a surface view of the postantennal 211) In some other Col- 

 organ. (After Willem.) O.ommatidium; Pa, post- 1 (* i -a -j 



antennal organ; /z3', hypodermal cells; iV, optic lemDOia, aS in Jinunaa 

 nerve; n, branch of the optic nerve; ^ /, tuber- mantima (Fig. 232, O), 

 cles surrounding the postantennal organ ;^g, the reduction of the om- 

 nerve-end-cellof the postantennal organ. (After rnatidia 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 

 Aniirida maritima by the same writer ('00). These organs were 

 termed the superlingucB 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), 



