ENTOGNATHY IN APTERYGOTES — TUXEN 



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cause they are a very characteristic feature of the slides and seem 

 to a large extent to be connected with what Snodgrass (1951) called 

 the midcranial ridge and which should not be confused with the epi- 

 cranial suture (both are indicated in figures 4 and 15). On the 

 pharynx are seen some muscles from the roof of the head. 



The mandibles of Collembola are also hollow at the inner and upper 

 side, but only to less than half their length apart from the teeth. No 

 rod bridges the cavity. A prostheca is not present nor any muscle 

 corresponding to b in Diplura. From the wall of the head just below 

 the antennae a process (fig. 8, x) meets a hump on the back of the 

 mandibles, thus yielding support to their movements. Also some 

 branches, y, from the epipharynx seem to support them at their in- 

 terior side. From the proximal end, which carries a condyle, a fanlike 

 ligament is seen running to the wall of the head (fig. 11) and forming 

 part of the limitation of the gnathal pouch; this ligament in certain 

 views may take the shape of a rod, but I do not think it is a real rod 

 (as drawn by Snodgrass, 1951, p. 85, fig. 31 G) and so I am not 

 convinced of its homologization with the rod of Chilopoda. 



A comparison with the muscles in Diplura has been tried (fig. 16) : 

 The two muscles c are strongly developed, whereas d is not found. 

 Two muscles, e, are present, crossing the middle line of the head and 

 attached to its dorsal wall on the other side. The straight muscles / to 

 the endosternum were not found, but only the oblique one g. Also 

 a muscle, probably corresponding to i, is present, though it does not 

 run to the hind wall of the head, but only to its dorsal wall somewhat 

 behind the attachment of e. And finally the muscles h which meet the 

 corresponding ones from the opposite mandible below the oesophagus 

 are not dispersed fanlike as in Camp odea, certainly because of the 

 much smaller cavity in Onychiurus. A comparison with Denis's figure 

 (1928, p. 112) would run as follows: c = I, e=A+B, g=IV-\-V, 

 h = VI, i=VIL In the figure, furthermore, some antennal muscles, 

 some characteristic cranial muscles, and some pharyngeal muscles 

 have been drawn. 



Denis (1928, p. 115) emphasizes the crossing of the muscles here 

 called c, found only in a few groups of insects. As an example he 

 mentions Campodea as figured by Nassonow (1887, Taf. i, fig. 4), 

 but that figure is incorrect ; as my figure 15 shows, there is no crossing 

 in Campodea, which as a matter of fact would seem to be prevented 

 by the midcranial ridge. 



The endosternum seemed visible to some extent in one of my slides 

 and has been outlined around the oesophagus. 



