Thanasimus formicarius
called the "myrbagge" in Swedish is the red-bellied clerid of Europe (family Cleridae, order Coleoptera). The beetle is about 1 cm long and rather soft-bodied but with large mandibles that can tear between the hard sclerotized integument of bark beetles. The adults overwinter at the base of pine trees and emerge in the spring and fly to downed pine trees in April (in Sweden). They disperse just before the flight of their prey (Tomicus piniperda and T. minor as well as other bark beetles) or during, or just after. Often they are waiting on the fallen pine trees and begin feeding on bark beetles as they land. Both the red-bellied clerids and the bark beetles are attracted to monoterpenes from the damaged areas of the fallen trees. Logging piles of trees are especially attractive due to the monoterpene volatiles. The clerid beetles are also attracted to specific pheromone components of bark beetles such as cis- and trans-verbenol and verbenone. In fact, the clerids are attracted to a wide range of compounds (Byers, unpublished).
The clerids grab their prey and bite off all their legs in some tens of seconds presumeably so they can't run away. Then the beetle bites between the thorax and abdomen, or head and thorax to leverage out the soft succulent parts of a bark beetle. The whole feeding process takes only about 10 minutes in the sunshine. The clerids can feed on several (about 3) bark beetles per day for several days. Occasionally a crippled bark beetle gets away or is dropped by accident. It is interesting that once a clerid begins to feed it almost always finishes the process so it never maims or wastes prey (a spritual idea almost).
As soon as a few bark beetles are eaten, however, the males begin searching for females and upon contact they immediately mate for a few tens of seconds, then the male "guards" the females by ridding on her back. But not too long and he is never aggressive to other wandering males (and vice versa). Many different males and females mate repeatedly (quite promiscuous) with the idea that the last male to mate inseminates a few eggs which the female lays in bark beetle holes so the larvae can get into the gallery system. New matings fertilize a few more eggs so the young of any female have many fathers. And of course each male has children with many different mothers. This is what is called "spreading the risk" in ecology supposedly so that inferior genes of one male do not monopolize the good genes of a female (i.e. a few children are incompetent but many have good qualities and are successful, especially important in unpredictable environmnets or all genes are not susceptible to a particular environment, so the theory goes). As far as I know it seems reasonable although other insects in the same environment do not have such a mating system but have only one or maybe two matings with males (or females).

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