Paussus favieri Fairmaire is one of only two species of the myrmecophilous carabid tribe Paussini known from Europe.
Larvae are known from only 10 of the 580 paussine species. As in many beetles with considerably modified later instar larvae, the
first instars represent a valuable source of informative characters for taxonomy and phylogenetic analyses (primary chaetotaxy, eggbursters, etc.). Therefore, the discovery of the first instar larva of P. favieri is particularly important, as it represents only the second
species for which this larval stage is known. In this paper we describe the behavior and morphology of the larval first instar of P.
favieri (subtribe Paussina of Paussini) and compare it with that of Arthropterus sp. (subtribe Cerapterina), which is the only other 1st
instar described in the Paussini. Most surprisingly, we found that the 1st instar of P. favieri lacks a prostheca, which was previously
thought to be a synapomorphy of Paussina + Platyrhopalina. Rather, P. favieri has a unique mandibular structure that seems to be
functionally analogous to the protheca. It is a long, broadly lanceolate, distinctly flattened structure apparently homologous to the
medial mandibular seta (MN2*), which arises from an area behind the cutting edge of mandible. We predict that the function of the
protheca and this similar structure in P. favieri are involved in a specialized feeding strategy that may include soliciting trophallaxis
from their host ants. We also report some observations of the first instar hatching from the egg, feeding on liquid and a behaviour we
interpret as a “calling behavior,” all of which were videotaped and posted on the Tree of Life Web Project.