COLLEMBOLA OF THE FAMILY ISOTOMIDAE d 



NOTES ON THE EVOLUTION OF THE MUCRONES 



A comparative study of the diverse forms of miicrones found in 

 Isotomidae leads to the inference that some of the forms have been 

 derived from others. The present account by no means exhausts 

 the subject. It consists simply of such conclusions as came to mind 

 during my systematic study of Isotomidae. 



A primitive type of mucro is found in Guthriella, essentially the 

 same form that occurs in several species of Achorutes} In Guthriella 

 vetusta, for example (pi. 10, fig. 104), the mucro is like that of Achorutes 

 socialis in its apical tooth, anteapical tooth, and outer and inner 

 lamellae extending from the anteapical tooth to the base of the mucro. 

 In Ballistura the mucro remains fundamentally as in Guthriella. 



With this generalized type of mucro as a basis, we may infer, by 

 comparing the various forms of mucrones with one another, that 

 specialization of the mucro has occurred in three main ways, as 

 follows: 



1. Lamellar development. The two primarj^ lamellae may increase 

 in size, and new lamellae may be added. Extreme conditions of 

 lamellar development are shown in schdferi (pi. 17, figs. 173, 174) 

 and rainieri (pi. 16, figs. 164-167). In the complex mucrones of 

 these species the apical tooth, anteapical tooth, and outer and inner 

 lamellae are evident, but additional lamellae are present, with addi- 

 tional teeth for their support. The extra lamellar expanse is corre- 

 lated with a superaquatic habit. 



2. Addition of teeth. The species titusi (pi. 18, figs. 186, 188) 

 shows the primitive pair of lamellae and the tv%'o primary teeth, with 

 an added third tooth, supporting a lamella. This third tooth, lateral 

 in position and anterior to the anteapical tooth, occurs in many 

 species of the family. 



The fourth tooth, as in commvna (pi. 17, fig. ISO) is never lateral 

 but is usually in line with the first and second teeth. In sepulcralis 

 the mucro, normally tridentate (pi. 20, fig. 213), is occasionally 

 quadridentate (pi. 20, fig. 214) as an individual variation. Many 

 other species have four teeth normally. The fourth tooth in jMlustris 

 (pi. 26, fig. 287) and in retardatus (pi. 25, fig. 279) supports a lamella. 



A few species, as rainieri (pi. 16, fig. 165), show a fifth tooth, near 

 the fourth. 



3. Lamellar reduction. Lamellate mucrones are characteristic of 

 the more primitive species of Isotomidae, and these species are rela- 

 tively few. In Proisotoma various stages in lamellar reduction are 

 exemplified in different species, but in Isotoma the mucrones are 

 typically without lamellae. As lamellae disappear the teeth that 

 supported them tend to remain, however, and in the nonlamellate 



'In the use of ^cftoTO^es instead oi ITypogastrura, opinion 65 of the International Commission on Zoolog- 

 ical Nomenclature has been followed. 



