1917] Chordotonal Organs of Cerambycid Larvce 67 



THE PLEURAL DISCS OF CERAMBYCID LARV^. 



We have already seen that the pleural discs of cerambycid 

 larvae are the outward expression of chordotonal organs. Since 

 these external structures of various larvae are being used as 

 systematic characters, we will consider briefly their arrangement 

 and relation in the different genera. In many species of the 

 Prionids, such as Mallodon dasystomus, we find a condition such 

 as is shown in Fig. 1, in which there is a pair of definite ray-like 

 structures on the pleural region of each of the first six abdominal 

 segments. These constitute the pleural discs (pi. disc) strictly 

 speaking. On the seventh and eighth abdominal segments they 

 are present, though somewhat modified. Instead of a simple 

 ray-like structure, we find here an elliptical enlargement which 

 Craighead calls the "Pleural tubercle" (PL Tu.). At the 

 posterio-dorsal side of this tubercle, the pleural disc can be 

 faintly seen (pi. disc). This ray-like structure or pleural disc, 

 has a small median depression, at the central point of which a 

 chitinous cap usually projects for a short distance. From this 

 median depression, there radiate folds in the body wall, pro- 

 ducing the previously described ray-like appearance. In addi- 

 tion to these ray-like folds, one often finds at the edge of the 

 disc, folds which run perpendicularly to the rays. 



Some of the smaller species of Prionids show the pleural discs 

 only faintly, and then usually best on the first three abdominal 

 segments. However, if one looks carefully, the others may also 

 be found. 



Outside of the sub-family Prioninae, all the species which the 

 writer was able to observe, possessed the elliptical or pleural 

 tubercle. In Saperda Candida, Rhagium lineatum, and Desmo- 

 ceriis palliatus, the elliptical enlargements were found on each 

 of the first eight abdominal segments, with a faint evidence of a 

 disc at the posterio-dorsal portion of each tubercle. In many 

 species a slight indication of a disc was also found at the anterio- 

 ventral end of the tubercle. 



In Monohammus confusor a very peculiar condition was 

 observed. The elliptical enlargement or pleural tubercle was 

 present, but instead of a ray-like structure, or pleural disc, at 

 either end there was found a depression in the form of a chit- 



