380 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I37 



entognathous apterygotes Insecta — a verbal and a technical problem: 

 must they be mentioned in entomological textbooks? The common 

 concept of an insect among laymen is that they are creatures with 

 six legs ; and the few instances of other arthropods with six legs 

 (among Acari and young myriopods), of course, do not alter this. 

 So the layman and the student will look for the three groups of 

 entognathous apterygotes in entomological handbooks, and they should 

 not be disappointed. The theoretical problem of the relationship of 

 entognathous apterygotes is, however, of much more interest ; Reming- 

 ton made a fine survey of the characters uniting or distinguishing 

 the groups of myriapods and insects. One of these characters I have 

 always found of very great importance, namely, the entognathy itself 

 in the three orders Protura, Collembola, and Diplura, because if it 

 is true that Collembola and Protura, far awa}^ from each other, branch 

 off at a "low" place on the "tree" of arthropods, and Diplura much 

 higher up, together with Symphyla, nearer the true insects, it means 

 that the character entognathy is polyphyletic. Remington (1954, 

 p. 499) puts it thus: "The phylogenetic significance of this condition 

 is obscure, but there seem to be excellent reasons for regarding its 

 origin as independent in each of these three groups. " 



What I wish to do in the following pages is to look a little deeper 

 into this question through a comparative analysis of the entognathy 

 in the three groups. I might have done it better by making sections, 

 which circumstances did not permit at the time, or by using a finer 

 technique ; I hope, however, to present a survey which may furnish 

 us with the means for a clearer insight. 



As to the technique, I shall mention only that I have treated the 

 insects (if I may be allowed to use this word) with lactic acid for a 

 shorter or longer time at different temperatures. Treatment for 24 

 hours at 55° C. will clear them up a little, at 80° C. much more, and in 

 both cases the muscles will remain, most visible in the latter case, when, 

 however, the mesodermal "tentorium" will be more or less dissolved. 

 Boiling the insects for only a few minutes in lactic acid will clear them 

 of all mesodermal tissue and leave the chitin untouched even in its 

 finest strands. By combining views of heads treated in these ways, 

 the accompanying figures were made. No staining was used. All 

 slides used for the drawings are kept in the Zoological Museum of 

 Copenhagen, so that the results can be checked at any time. 



Now, the basic question of course will be: how is entognathy to 

 be understood? On this point Denis (1949, p. 112) writes: "L'ento- 

 trophie ne resulte pas de I'enfoncement des gnathes mais de leur re- 



