30 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



each other at the intersegmental sutures. The tergum of the prothorax 

 is without apodemes or sutures, but each of the other two possesses 

 three longitudinal internal ridges (pi. lo, fig. 30). One of these is 

 straight and median ; the other two are lateral and outwardly curved. 

 In Japyx bidcntatus the median ridge is wanting, but a transverse 

 groove divides the tergum proper from an anterior region termed the 

 precosta by Snodgrass ( 1927) . This area is very large in Japyx bidcn- 

 tatus, being about one-fourth as long as that of the remaining part of 

 the tergum. 



In the Collembola the thoracic terga extend far down on the sides 

 of the body, reaching in some instances the bases of the legs (pi. 10, 

 fig. 31). The tergum of the prothorax is always reduced and may be 

 almost obliterated in certain genera. In Isotoma only two minute, 

 transverse strips of chitin remain as vestiges of the first thoracic 

 tergum. In general the collembolan thoracic terga may be said to be 

 of the simplest type, being without apodemes or sutures. 



CONCLUSIONS REGARDING THE TERGA OF LEG-BEARING SEGMENTS 

 OF PRIMITIVE ARTHROPODS 



The prevailing type of terga found on the leg-bearing segments of 

 Pauropoda, Symphyla, Protura and Apterygota is the simplest type 

 known — a tergum without apodemes, sutures or specialized areas 

 of any kind. This is the type found in the larvae of many insects. 

 The presence of an anterior, transverse apodeme dividing the tergum 

 into a very small anterior area and a very large posterior one is 

 noted in the Protura and the Thysanura. This area, which has been 

 termed the precosta, and the apodeme which has been termed the 

 antecostal suture, appear to be structures fundamental to the terga 

 of winged insects. 



THE STERNAL PLATES OF THE LEG-BEARING SEGMENTS 

 STERNA IN ARACHNIDA 



In the more generalized arachnids like the Solpugida, Cyphoph- 

 thalmi, and Pedipalpida, sternites are frequently wanting from some 

 or all the cephalothoracic segments. In such cases the basal seg- 

 ment of the leg, the subcoxa, usually is flattened and more-or-less 

 platelike. These subcoxae are liable to be fixed and fused each with 

 its fellow of the opposite side along the median plane. Such a con- 

 dition is found in the Cyphophthalmi (pi. 11, fig. 32). In the Pedipal- 

 pida, as illustrated by Mastigoproctus gigantcus, the giant whip- 

 scorpion of the South, the first and last cephalothoracic segments are 



