Predaceous Diving Beetles

Dytiscidae. Spanish: Escarabajos acuáticos depredores (General).

This and the following are the most common families of beetles leading submerged lives in all aquatic habitats, for example, ponds, streams, vegetated lake margins, marshes, even hot springs.

Dytiscids (Moroni and Bachmann 1977; Spangler 1981, 1982) are broad but streamlined, boat-shaped beetles. Like water scavenger beetles, they have flattened hind legs fringed with hairs, but the threadlike antennae are much longer than the palpi. When swimming, the hind legs move together, oarlike; hydrophilids move them alternately. They are usually brown or black, but some have yellow or whitish spots or other markings. There are over 550 Neotropical species. Most are small to medium (BL 1.5—15 mm), but there are large (BL to 35 mm) species in the genera Cybister (fig. 9.3a) and Megadytes (fig. 9.3c). These are black to dark olive green and often have a wide buff to yellow marginal border. The elytra may be smooth or deeply striated.

All active stages are highly predaceous; the larvae (fig. 9.3b), known as water tigers, have pronounced, channeled, sicklelike jaws with which they grasp prey and through which they suck its body juices. They lack abdominal gills but have a pair or three long apical appendages. Adults of many species produce milky secretions containing defensive steroids from glands on the prothorax (Miller and Mumma 1974).

These beetles do not have the ventral film of air held by hydrophilids, being almost hairless underneath. Because the major spiracles are on the abdomen, dytiscids come to the surface for air with the tail up.

References

Miller, J. R., and R. O. Mumma. 1974. Seasonal quantification of the defensive steroid titer of Agabus seriatus (Coleóptera: Dytiscidae). Ento-mol. Soc. Amer. Ann. 678: 850-852. Moroni, J., and A. C. Bachmann. 1977. Dytiscidae. In S. H. Hurlbert, ed., Biota acuática de Sudamérica austral. San Diego State Univ., San Diego. Pp. 217-225.

Figure 9.3 WATER BEETLES, (a) Predaceous diving beetle (Cybister sp., Dytiscidae). (b) Predaceous diving beetle, larva, (c) Predaceous diving beetle (Megadytes giganteus, Dytiscidae). (d) Water scavenger beetle (Tropisternus lateralis, Hydrophilidae). (e) Water scavenger beetle (Berosus sp., Hydrophilidae). (f) Giant water scavenger beetle (Hydrophilus insularis, Hydrophilidae). (g) Whirligig beetle (Gyretes sp., Gyrinidae). (h) Giant whirligig beetle (Gyrinus sp., Gyrinidae).

Figure 9.3 WATER BEETLES, (a) Predaceous diving beetle (Cybister sp., Dytiscidae). (b) Predaceous diving beetle, larva, (c) Predaceous diving beetle (Megadytes giganteus, Dytiscidae). (d) Water scavenger beetle (Tropisternus lateralis, Hydrophilidae). (e) Water scavenger beetle (Berosus sp., Hydrophilidae). (f) Giant water scavenger beetle (Hydrophilus insularis, Hydrophilidae). (g) Whirligig beetle (Gyretes sp., Gyrinidae). (h) Giant whirligig beetle (Gyrinus sp., Gyrinidae).

Spangler, P. J. 1981. Dytiscidae. In S. H. Huribert, G. Rodriguez, and N. Dias dos Santos, eds., Aquatic biota of tropical South America. Pt. 1. Arthropoda. San Diego State Univ., San Diego. Pp. 136-148. Spangler, P. J. 1982. Dytiscidae. In S. H. Hurlbert and A. Villalobos Figueroa, eds., Aquatic biota of Mexico, Central America and the West Indies. San Diego State Univ., San Diego. Pp. 335-343.

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