357 

 Leech and Chandler: Coleoptera 



ing to vegetation. Adult elmids are small, compact 

 beetles with an average size of only 2-3 mm. The 

 color of the dorsal surface may be black, gray, brown, 

 or red, with the ventral surface clothed with pile or 

 tomentum so that it usually has a silver gray color. 

 Like the Dryopidae, they have the last tarsal segment 

 and claws greatly enlarged for anchoring themselves 

 to the substrate, and the head may be retracted into 

 the prothorax, concealing the mouth parts. The larvae 

 are elongate, cylindrical, hemicylindrical or sub- 

 triangular in cross section, with the head conspic- 

 uously narrower than the thorax and the abdomen 

 tapering posteriorly. The last abdominal segment has 

 a ventral caudal chamber, closed by an operculum, 

 into which can be retracted two prehensile hooks and 

 three tufts of filamentous gills. 



Relationships. — The Elmidae are closely related to 

 both the Dryopidae and the Limnichidae. General body 

 form closely resembles many of the dryopids, but the 

 antennae are never pectinate. There are also numerous 

 differences in the internal anatomy. The larvae are 

 quite different. Adults of the Limnichidae do not 

 closely resemble elmids, but the larvae are quite 

 similar. 



R espiration. —The method of respiration of adults is 

 similar to that discussed in the section on Dryopidae. 

 All elmid larval instars except the first have nine 

 pairs of more or less evident spiracles situated on 

 the mesothorax and first eight abdominal segments. 

 These are reported by Susskind not to be connected 

 with the tracheae until the last larval instar. Respi- 

 ration in the water is apparently accomplished entirely 

 by means of the three tufts of retractile gills situated 

 in the caudal chamber. The number and length of these 

 gill filaments will vary with the different species. 

 Certain species which live in fast, cold water have 

 been noted to have shorter or fewer gills than normal. 

 The larva of Dubiraphia which has the last abdominal 

 segment much longer than in the other genera, also 

 has the caudal filaments two to three times as long 

 as in most genera. This may explain why Dubiraphia 

 is one of the few genera to inhabit lakes and slow 

 moving rivers. Larvae have been observed to expand 

 and contract their filaments rhythmically when the 

 oxygen balance of the water is reduced. 



Life history. — The female genitalia are not adapted 

 for inserting the eggs into plant tissue as are those 

 of Dryopidae. It has been suggested that a life cycle 

 may take two years in this group because adults and 

 larvae of the same species were taken at the same 

 time. However, in California as many as five size 

 groups of larvae, as well as adults, have been taken 

 by processing stream screenings through a Berlese 

 funnel. When mature the larvae crawl out of the water 

 and pupate under stones on the bank. The few species 

 that have been reared usually emerged as adults in 

 about two weeks. Both larvae and adults feed upon 

 algae, moss, and other vegetative matter in the water, 

 including the roots of larger plants. 



Habitat and distribution. — Elmids are found in 

 streams of all types but may be rare in those with 

 seasonal flow, heavy sediment load, mud or sand 

 bottoms, low gradient, or low oxygen content. Only a 



Fig. 13:51. Elmidae. a-d, ventral view of thorax and first 

 abdominal segment; o, Cylloepus sp.; fa, Heferelmi s sp.; c, 

 Elsianus sp. (second abdominal segment shown here); d, Micro- 

 cylloepus sp.; e, Ampumixis dispar, adult pronotum; f, Oplio- 

 servus sp., larval mesothorax, ventral; g, Heterlimnius sp., 

 larval prothorax, ventral; h, Ordobrevia sp., larval mesothorax, 

 ventral (a-d, Hinton, 1940; e,h, Sanderson, 1953; i,q, Sanderson, 

 1954). 



few genera are found in lakes, ponds, or slow moving 

 streams. The most common in California is Dubi- 

 raphia. Micro cylloepus may be found along rocky shore 

 lines where wave action aerates the water. Microcyl- 

 loepus moapus, M. thermarum, and Stenelmis calida 

 are all restricted to warm springs in Nevada. Heter- 

 limnius koebelei Martin has been collected at altitudes 

 up to 8,000 feet in California. Adults of Lara are not 

 aquatic, but are found under logs and trash at the 

 water's edge. They are more active than those of 

 other genera and will quickly hide if exposed, even 

 scrambling over the surface of the water to reach 

 shore. Most elmids are very lethargic and will play 

 dead for some time after" being exposed. Cleptelmis 

 may not start to emerge from masses of moss placed 

 in a Berlese funnel for twelve to fifteen hours. 



Elmids are found throughout the tropical and tem- 

 perate regions of the world. In North America they 

 extend from Mexico through the United States into 

 southern Canada. I have seen larvae from as far north 

 as Great Slave Lake. In general, the Nearctic genera 

 may be placed in six distribution groups. The genera 



