Adapting to cope with eucalypt oils: mandibular extensions in pergid sawfly larvae and potential preadaptations in its sister family Argidae (Insecta, Hymenoptera, Symphyta).

Each of the interior mandibular surfaces of Australian sawfly larvae in the subfamily Perginae is equipped with a soft, brush-like scopa mandibularis. These insects are associated with oil-rich Myrtaceae, including Eucalyptus, and the scopa is involved in separating leaf oils from nutritive plant matter. The oil is stored internally in a diverticulum and is emitted during defense reactions. However, this is known only from mature larvae and the mandibular structure has been investigated and partially illustrated in only one genus of pergines, Pergagrapta. Here, we provide a full description, extend this to a second genus, Perga, and include first instar larvae of Perga and Pseudoperga genera to confirm the presence of the scopa and diverticula through the entire larval life, and thus infer their developmental trajectory. Superficial descriptions of mandibular projections in some Nearctic species in the family Argidae, which is phylogenetically sister to the Pergidae, have been published. Modifications of the inner mandibular surface of representatives of the argid genera Sericoceros, Sphacophilus, and Zynzus are therefore fully illustrated and compared with those of the Pergidae, so this phylogenetic relationship can be assessed.