COLLEMBOLA OF MINNESOTA 11 



evidently only a modified pair of jointed appendages which have 

 taken the function of holding the furcula in position when not 

 in. use. As a fore-runner of the ventral tube in Collembola, 

 we have the exsertile vesicles in Machilis in which they are 

 found at the bases of the legs. Perhaps the common ancestor 

 had them on all the appendages, or at least on those of the 

 first abdominal segment, and that this pair persisted and moved 

 toward the median line until they grew together and formed 

 one tube. 



That the Collembola are extremely old is indicated by 

 the fact that, despite their wingless condition, the same species 

 with almost no variation will occur often throughout three con- 

 tinents. Dr. Folsom considers that this remarkable distribu- 

 tion has been effected by the inland streams and ocean currents, 

 which latter are known to convey inland species to great dis- 

 tances. 



Many considerations, on the other hand, point to their 

 being a regressive branch ; such, for instance, are furnished by 

 their retiring, dark-loving habit, and minute size. The habit 

 mentioned is, in turn, probably responsible for the tendency 

 noticed in most of the genera toward a loss of the eyes. In 

 isolated species in several genera we find all or nearly all pig- 

 ment in the body lost, and the occeli entirely atrophied, owing, 

 probably, to the cave-habit. Worthy of note, along with the 

 assumption of the cave habit and consequent degradation and 

 atrophy of optical sense, has come an hypertrophy of senses 

 along other lines: at least this is the obvious way of accounting 

 for the abundance of multiform tactile hairs in positions where 

 the touch power could be best utilized. Whether we are to rec- 

 ognize merely a protective function in the scales which adorn 

 Tomocerus and Seira, etc., or whether they also act as sense 

 media, is a matter for further investigation. 



No small part of the difficulty arising in the study of this 

 order is due to their diminutive size. Not only does their minute- 

 ness, coupled often with extreme agility, render them difficult to 

 observe and still more to capture ; but it is often no easy matter to 

 make out such small microscopic characters as the form and dis- 

 position of teeth on the claws and furcula. Remembering that 

 several species never attain more than a half millimeter in length, 



