4 BULLETIN 168, UNITED STATES NATIONAL MUSEUM 



species of Isotoma the third and fourth teeth are often present as 

 vestiges. 



Variations. — Though the mucrones in Isotomidae have assumed 

 many different forms, there are few species in which the two primary- 

 teeth — apical and anteapical — are not present. With the reduction 

 of the furcula they still remain, as in Folsomia and other genera. 

 Moreover, they persist in most Entomobryidae. Mechanically, these 

 two are the most important teeth in the act of springing. 



The teeth of the mucrones vary greatly in size and form. The 

 apical tooth may enlarge and elongate, as in albella (pi. 31, fig. 359), 

 marissa (pi. 29, fig. 327), and other species. On the other hand, it 

 may become reduced partially as in arborea (pi. 28, fig. 312) and in 

 grandiceps (pi. 36, fig. 426) ; or until it is merely a minute tooth at the 

 base of the anteapical tooth, as in palustris (pi. 26, fig. 287). 



The third, or lateral, tooth in titusi (pi. 18, figs. 187, 188) and other 

 of the more generalized species supports a lamella, but it has persisted 

 in many species after the lamella has disappeared. It is strongly 

 developed in trispinata (pi. 29, fig. 333) and marissa (pi. 29, fig. 327). 



The form of the mucro varies not a little among individuals of the 

 same species and even in different instars of the same individual. 



PHYLOGENETIC AND TAXONOMIC NOTES 



Isotomidae are generally regarded as having originated from forms 

 resembling Achorutes,^ which in turn appear to have come from ances- 

 tors like Podura, with a well-developed furcula (Willem, 1900). 



The much-discussed Tetracanthella and genera allied to it are 

 accepted as links between Isotomidae and Achorutidae.^ Even better 

 links, however, between these two famihes are found in the species of 

 Guthriella that are treated in this paper. 



The habitus of Guthriella is like that of AcJwrutes. The body, legs, 

 and furcula are stout. The body segments are bulging, with deep 

 intersegmental constrictions, with very narrow intersegmental mem- 

 branes, and without imbrication (except between the genital and anal 

 segments in one species). The integument is tuberculate throughout 

 but soft, the body sclerites being indicated but not sharply bounded. 

 The prothorax is exposed, tuberculate, and pigmented, and in 

 muskegis it bears a pair of setae. The third abdominal segment is a 

 relatively simple ring. The abdomen is without a median ventral 

 groove. The third antennal segment bears a pair of subcyhndrical 

 sensory rods, which in antiqua are curving, subtended by a chitinous 

 ridge and accompanied by two accessory rodlike setae, the whole 

 organ being almost exactly like that of Achorutes socialis (Folsom, 

 1916,pl.ll,fig.47). The fourth antennal segment has stout, curving, 



'See footnote I, p. 3. 



