6 COLLEMBOLA OF MINNESOTA 



through in the degeneration of compound eyes might appear 

 not unlike the typical collembolan eye. If, as I have supposed, 

 the Machilis is very like their ancestor, then we must conclude 

 that the eye patches with their grouped ocelli represent the orig- 

 inal compound eyes. The degeneration in the number of ocelli 

 has already been noted. 



The antennae vary greatly in their comparative length and 

 segmentation. We may regard four as the normal number of 

 segments ; the proximal segment being usually short, and very 

 often the second segment as well. The two distal segments 

 differ among different genera. In Papirius and Tomocerus the 

 end segment is short and ringed with whorls of short hairs, as 

 is also at least a part of the third segment. In Sminthurus only 

 the fourth is ringed, and that not always. The antennae are im- 

 portant tactile organs, as one will soon discover by watching 

 these creatures moving about and keeping the antennae in con- 

 stant motion. The antennae are always haired, but, besides the 

 ordinary hairs, special long, slender hairs are often borne stand- 

 ing out at nearly right angles to the members which bear them. 

 In some of the lower genera, as among the Achorutes and Ne- 

 anura, where the habit is less active, there appears to be a cor- 

 responding degeneration of the antennae ; the third and fourth 

 segments being sometimes reduced to one. Whether the ringed 

 condition of the antennae of such forms as Tomocerus points to 

 a primitive multiarticulate condition as in Machilis is a matter 

 of doubt. While it is possible, yet I incline to think rather that 

 the partial sub-jointing has been developed in the Tomocerus 

 and Sminthuridae independently in response to a demand for 

 more flexible tactile organs. 



The post-antennal organ is characteristic of some of the 

 genera, yet can hardly be regarded as belonging to the group 

 as a whole. It will be noticed under the genera where it occurs. 



THORAX. The thorax is typically that of a hexapod. 

 Three free segments bear each a pair of typical walking legs. 

 In the Sminthuridse these segments are fused together. 

 Usually the prothorax is the smallest of the three, and in sev- 

 eral of the genera the tergttm of the following segment pro- 

 jects over it to a greater or less extent. The fact that the meso- 

 and metathorax tend to be larger than the prothorax has lent 



