The Carlisle Mosquito Online

Friday, January 25, 2008


Snail-eating Beetle

Snail-eating Beetle.

Stag Beetle. For comparison, the stag beetle's antennae have distinct "elbows" and open out at the tips, quite different from the slender antennae of the Snail-eater. Photos by Kay Fairweather

On January 12 the daytime temperature was in the low to mid 40s. Much of the December snow had melted and the next snow had not arrived. I was out walking with Mary Zoll when she spotted a black beetle crossing the road on School Street. (I think it thought it was a chicken.) I don't usually see beetles in the winter time, even on mild days, except by looking under logs or loose bark or similar places that offer a lot of protection. Anyway, it was an interesting-looking fellow and Mary let me take it home. I kept it in a dish for several days while trying to identify it and then set it free again in a safe place in my neighbor's yard. (He is happy to give it a home — it's a beneficial beetle.)

Identification. I was able to identify the beetle as a member of the Carabidae family also known as Carabids or ground beetles (that would not be ground as in ground round but ground as in ground-dwelling). The structure of beetle antennae is very helpful in identification. This beetle had rather long thread-like antennae with 11 cylindrical segments of approximately equal length, which helped me arrive at the Carabids. Peter Messer and Eric Eaton, experienced entomologists at, were able to identify it as a Snail-eater in the genus Spaeroderus and probably S. stenostomus. (This is the same Eric R. Eaton who is the principal author of the excellent Kaufman Field Guide to Insects.) The beetle was black with blue iridescent edges on the thorax and abdomen and was just over half an inch long.

Mouthparts. The head was narrow and the jaws or mandibles were prominent but not as fierce-looking as on a tiger beetle (also in the Carabid family) or a stag beetle. The Snail-eating Beetle also had two pairs of quite distinctive mouthparts. You can see the longer of the two pairs in the photo, extending out in front of the mandibles. These mouth parts are called palps and they help with both detection and manipulation of food. Chemo-receptor cells on the palps of Snail-eaters are massively more numerous than those on beetles with a less specific diet. While the Snail-eating Beetle can use its narrow head and palps to get right inside the shell of small snails, it will also eat other little critters if escargots are scarce.

Flightless. Lots of ground beetles don't fly. Some can't. When threatened, they run. I kept this beetle in a plastic dish about an inch deep with no cover. It was very healthy and active but showed no signs of trying to fly out.

An inconvenient beetle. It is not typical for Snail-eaters to be out and about in mid-winter in Massachusetts. Peter Messer recommended that I save field notes of sightings of this kind in case they should ever be collected for "a possible sequel to a famous movie documentary."

Sources. (an excellent web site for insects and for ID assistance); this beetle, actually this specimen, has a page at; Borror and Delong's Introduction to the Study of Insects, Charles A. Triplehorn & Norman F. Johnson.

2008 The Carlisle Mosquito