520 
Psyche 
[September-December 
(Fig. io). A considerable quantity of air is trapped among these 
overlapping, elongated setae. On living specimens the sense organs 
are the only parts of the body on which air can be seen with the 
naked eye (Polhemus, personal communication), and the air on these 
regions is easily seen with the stereoscopic microscope. 
In A phelocheirus similar sense organs, which are probably pressure 
receptors, occur only on the second abdominal segment (Thorpe and 
Crisp 1947c). Like those of Cryphocricos they are composed of 
flattened, elongated setae, approximately 75 /mi long, which point 
posterolaterally and overlap each other. The setae appear to be longi- 
tudinally ridged, although this could not be determined with certainty 
because of the large amount of debris which covered them. Leaf-like 
setae were not observed on any other region of the body of A phelo- 
cheirus. 
( 3 ) M icrotrichia 
O11 the anteriormost portions of the fourth through sixth abdomi- 
nal sternites (Fig. 1, ST 4-ST 6) plastral hairs occur only laterally ; 
near the midline they are lacking. The only structures observed in 
this hairless region are short, blunt microtrichia, approximately 1 /mi 
wide at the bases and 2 /mi long, which project posteriorly. They are 
arranged in widely-spaced groups of from two to six. Each group of 
microtrichia arises from the slightly raised, curved, posterior edge of 
a roughly semicircular region of the cuticle. The microtrichia lack 
sockets and lie nearly parallel to the surface of the cuticle. 
More posteriorly on the fourth through sixth sternites, but not as 
far as the posteriormost edge, the microtrichia are more numerous 
per group, longer (up to 3 /mi), and occur in more closely-spaced 
groups (Fig. 11). Each group consists of 5-15 microtrichia and 
these, along with the region of smooth cuticle to which they attach, 
form a fringed, semicircular unit. The units are arranged like shin- 
gles. On this central part of the sternite, unlike the more anterior 
region, hairs appear to be present between adjacent rows of micro- 
trichia. Since only the tips of these hairs are visible, we could not 
determine whether they are similar to the short plastral hairs which 
cover the lateral and posterior regions of the fourth through sixth 
sternites; these regions lack microtrichia (Fig. 1, unstippled portions 
of ST 4-ST 6) . 
(4) Fringed Setae (Fig. 12) 
Fringed setae may occur either singly or in large numbers. They 
arise from sockets in the cuticle and project posterolaterally. A trails- 
