no. 3646 DINEUTUS — WOOD 7 



The two pygidial sacs lie laterally between the caudal abdominal 

 segments and release a white liquid when the live insects are dis- 

 turbed. The effect of alcohol or drying on this exudate are unknown 

 but the pygidial sacs turn white. This color may be altered or masked 

 outwardly by pigment in the body fluids; nevertheless, it shows 

 through unpigmented or lightly pigmented integument. Where the 

 integument is very darkly pigmented, like the piceous portion of the 

 abdomen of D. s. analis, internal tissues are masked completely; but 

 where pigmentation is light or lacking (e.g., portions of the last three 

 ventral abdominal segments), tissues show through, presenting a 

 light color. In the uniformly dark specimens from Hope, Ark., there 

 is no absence of pigment in the caudal ventral segments, and all the 

 ventral sterna are pigmented equally. The head, pronotum and elytra, 

 and terga of the last three abdominal segments are pigmented heavily 

 and darkly, but the thoracic sterna are pigmented like the abdominal 

 sterna. The heavy and broadly attached thoracic muscles do not 

 separate easily from the ventral integument; furthermore, they fill the 

 thoracic cavity so completely that there is little change in ventral 

 thoracic coloration regardless of treatment. The actual piceous 

 pigmentation of most of the venter of D. s. analis, therefore, allows 

 for little change in ventral coloration, except caudally, while the more 

 lightly pigmented venter of D. s. serrulatus permits variation in color, 

 depending on how the specimen has been treated. 



The protibiae of D. s. serrulatus are rounded at the exterior apical 

 angle. This character, as with the western subspecies, is more evident 

 in the male but is constant in both sexes. The dorsum of the eastern 

 taxon is usually black with an occasional bronze form occurring within 

 a series. All specimens in both subspecies that are black dorsally 

 show very faint bronzing under magnification, especially when viewed 

 laterally and more especially on the lateral aspects of the pronotum 

 and the head. The specimens studied (over 900) ranged from 9 to 12 

 mm in length (measured from the anterior margin of the clypeus to 

 the elytral apices). 



Literature Cited 



Ahlwarth, K. 



1910. Gyrinidae. In Coleopterorum catalogus, pt. 21, pp. 1-42. Berlin: 

 W. Junk. 

 Blatchley, W. S. 



1919. Insects of Florida, Va: Supplementary notes on the water beetles. 

 Bull. American Mus. Nat. Hist., vol. 41, art. 4, pp. 305-322. 

 Brimley, C. S. 



1938. Insects of North Carolina, 560 pp. 

 Hatch, M. H. 



1925. An outline of ecology of Gyrinidae. Bull. Brooklyn Ent. Soc, vol. 20, 

 no. 3, pp. 101-114. 



