THE CLASSIFICATION OF INSECTS 



155 



known as the spring-tails, or Collembola, are very small indeed, 

 most of them measuring only two or three millimeters in length. 

 These Collembola are more specialized in structure than the 

 other Aptera, and represent a side line of evolutionary de- 

 velopment within the order. Most of them possess a curious 

 forked spring on the under side of the body by means of 

 which they leap vigorougly when disturbed. But few of 

 these minute insects are injurious, although at least one spe- 

 cies, called the garden-flea, S my nt hunts hortensis, is sometimes 

 found in considerable numbers 

 upon the leaves of young cab- 

 bages, turnips, cucumbers and 

 various other plants, on which 

 it probably feeds. Certain 

 other species are reputed to 

 exist in such abundance in the 

 soil of flower and vegetable 



FIG. 67. The spotted spring- 

 tail, Papirius maculosus, with 

 spring extended. (Natural size, 

 two millimeters.) 



FIG. 68. Young and adult Lc- 

 pisma sp. from California. (Twice 

 natural size.) 



beds as to keep the soil so constantly disturbed by their 

 movements that the roots cannot hold the plants firmly. 



The other principal group in the order, known as the Thy- 

 sanura, is represented in this country by three small families 

 which contain but a few species. All the Thysamira have a soft 

 flattened body of from but a few millimeters to three-fourths 

 of an inch in length, and live mostly under stones and logs in 

 the soft soil and humus at the bases of tree trunks. A common 

 species that occurs in houses is known as the silver-fish, or 

 fish-moth, Lepisma saccharina. It is about one-half an inch 

 long and silvery white with a yellowish tinge. It feeds 



