In Peritrechus, the structure of the antennae is an important diagnostic character. In Peritrechus geniculatus, ant3 is as thick or thicker than ant1, whereas in the other very similar species it is thinner. This recent specimen was missing the left ant4 but still identifiable as P. geniculatus. Then Jim Flanagan pointed out something odd to me. Maybe the left ant4 never got lost - it just didn't grow? The key is that the left ant2 is longer than the right - this is called oligometry and is frequent in Lygaeidae. Oligometry usually results in one of the antennae having three segments (it can also affect both antennae and then it is known as bilateral oligometry). It is thought to be caused during late development phase but precise reasons behind this deformation are not well known.
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