Identification, Images, & Information
For Insects, Spiders & Their Kin
For the United States & Canada

"Riffle beetle" - Stenelmis
Medford, Burlington Co. (~15 miles east of Philadelphia, PA) County, New Jersey, USA
July 11, 2006
Size: ~3mm
came to UV light trap. I haven't seen these before at my place, but tonight they came by the thousands - probably even out numbered the carabids. The other family common name, Marl Water Beetles, would better suit this fellow, as swiftly flowing streams with lots of riffles are not exactly common in my part of (flat, tidal) south Jersey. But wetlands with lots of stagnant surface water over top of marl clay certainly is the norm. I'm only a few miles from the town of "Marlton". This Elmidae is probably Stenelmis quadrimaculata

Moved from Stenelmis.

Moved - Thanks Crystal
Moved from Stenelmis sinuata.

Be careful! This specimen is
Be careful! This specimen is not Stenelmis sinuata. Stenelmis sinuata has testaceous maxillary palps and antennae and this one has black antennae (well, actually they are kind of two-toned). Also, the locality for this individual is well out of the range of Stenelmis sinuata (it's a southern species) and it is much more likely to be either S. musgravei or S. quadrimaculata. Again, it's tough to say which without genitalia, so I'd say just keep it at Stenelmis sp.

Missed this comment...
thanks for the correction...


Moved from Riffle Beetles.

rather, Stenelmis crenata
upon further review, this is probably Stenelmis crenata (the 5th tarsi seems clearly longer than others combined)

Probably quadrimaculata
You had it right the first time, well actually, you also had the key right too, just the wrong species. The Humerosa-sinuata group (which S.quadrimaculata is in) has the 5th tarsi distinctly longer than the rest combined. The Crenata group, on the other hand, has the 5th tarsi that are NOT distinctly longer than the rest combined. This is most likely S. quadrimaculata,judging from where you found it. However, The key couplet that splits S. quadrimaculata from S. musgravei involves genitalia, so you won't be able to get a definitive answer without pulling the genitalia out. Try using

Brown,H.P. 1976. Aquatic dryopoid beetles (Coleoptera) of the United States. EPA Water Pollution Control Research Series

I hope that this helps!

thanks Crystal
I was keying using Downie and Arnett's Stenelmis key which they state "from Sanderson, 1938". It appears the first cuplet may be reversed or at least inaccurate. 1a: "last tarsomere distinctly longer than 4 preceding combined, last usually abruptly dilated beyond middle; claws robust." leads to; sexlineata, crenata, lateralis, concinna, sandersoni, mera, and bicarinata.

I found your Brown '76 reference on EPA's site and posted the link on the Elmidae family Guide info page.

Given all this, would you please check my image post for S. bicarinata to see if I missed this ID too?