FOLSOM: MOUTH-PARTS OF ANURIDA MARITIMA. 141 
diimnen Chitinstab, welcher oben im Kopfe, hinter den Augen, endet 
und da am Chitin des Kopfes festsitzt.”’ 
Thus, the tentorium of Machilis is constructed upon the same plan as 
that of Anurida, although the median plate is halved longitudinally. 
The dorsal and posterior arms in Anurida are clearly represented in 
Machilis, and the latter pair tends to become reduced in size, — an 
approach to the Orthopteran condition. 
The tentorium in Orthoptera is readily comparable with that of 
Machilis. In Periplaneta, according to Miall and Denny (’86, p. 39), 
“Tn front it gives off two long crura, or props, which pass to the gin- 
glymus, and are reflected thence upon the inner surface of the clypeus, 
ascending as high as the antennary socket, round which they form a 
kind of rim. Each crus is twisted, so that the front surface becomes 
first internal and then posterior as it passes towards the clypeus. The 
form of the tentorium is in other respects readily understood from the 
figure (Figure 17). Its lower surface is strengthened by a median keel 
which gives attachment to muscles. The esophagus passes upwards 
between the anterior crura, the long flexor of the mandible lies on each 
side of the central plate; the supracesophageal ganglion rests on the 
plate above, and the subcesophageal ganglion lies below it, the nerve- 
cords which unite the two passing through the circular aperture. <A 
similar internal chitinous skeleton occurs in the heads of other Orthop- 
tera, as well as in Neuroptera and Lepidoptera.” 
In Anabrus (Packard, ’98, p. 49, Figure 33) the tentorium is essen- 
tially the same, with a central plate, and paired dorsal and posterior 
arms. The only important differences between Orthoptera and Collem- 
bola in respect to the tentorium are (1) that the cesophageal commissures 
pass through it in the former group instead of around it; (2) that in 
Orthoptera the posterior arms are reduced in length, and (3) that the 
tentorium becomes .more stoutly chitinized. On the other hand, the 
tentorium of Orthoptera, in its general form and topographical relations, 
agrees closely with the same structure in Collembola and Thysanura. 
Palmén (’77) derived the tentorium from a pair of cephalic trachez in 
Ephemera, but upon unsatisfactory grounds. In Collembola trachez 
are absent ; moreover, as Packard (’98, p. 50) notes, ‘the apodemes of 
the thoracic region are evidently not modified tracheze, since the stigmata 
and trachez are present.” 
The views of Carriére (90) and Cholodkowsky (91), agreeing with 
the opinion of Palmén, have been controverted by Heymons (’95°, 
pp. 50-51). 
