Fig. 20. When there are many bedbugs, it is sufficient to see their excrements if you look behind the pictures and behind loose wallpaper. Bed bug excrements are black, have a characteristic shape (right) and do not look like the spots that are made by spiders. (Diehl & Weidner and Kemper)

It is rare to find a lonely bed bug, when it is not just a case of an individual who has ventured forward to search for blood. They have a pronounced desire to clump together.

If you have found one of their strongholds, and it has been in use for a long time, you will find a jumble of adults of both sexes, nymphs, empty exoskeletons, eggs and empty egg shells and then the black, blood-containing excrement (fig 20). Their unity is partly because of their looking for body contact with each other, but especially because of a scent – again a pheromone – they release which is appealing to similar species.

It is the same pheromone that leads them back to the hideout when they have been out on the hunt for blood. The smell is not pleasant in our nostrils.

Johannes V. Jensen is very well exaggerating when he in one of his “Myths” writes: “One could smell like a world of fusel and poverty Pjaltenborg fuming out of the tiles of dung and spirit”. A more sober description is that the smell is similar to rotting raspberries. With great success in certain parts of the world, so called bed bug dogs that were trained to smell for bed bugs have been used.