Aphagy and vestigial stylets in first-instar nymphs of Aradidae (Hemiptera, Heteroptera).

Aradidae are known for their remarkably long stylets, coiled at rest in the anterior part of the head. However, previous reports indicated that at least some species lacked stylets during the first nymphal instar. A more detailed examination of Aradus betulae 1st-instar nymphs showed that their mandibular and maxillary stylets are abnormally short, not coiled, improperly interlocked, and clearly non-functional. The anteclypeus is relatively small and its internal diverticulum, which accommodates the stylet coil in the older stages, is vestigial. In contrast, the labium, labrum, food canal, and associated structures and muscles, including protractors and retractors of the stylets, are all normally developed. First-instar nymphs of Aradidae are the first known Heteroptera with non-functional mouthparts. To explain this phenomenon, a hypothesis is proposed which links previously unexplained records of non-feeding (but endowed with regular, functional mouthparts) 1st-instar nymphs of various pentatomomorphan families with the special role of that stage in acquiring microbial gut symbionts. A presumed loss of symbionts in the ancestors of Aradidae may have led to reduction of the now useless stylets in the first instar, which retained aphagy, despite a spectacular elongation of stylets in the older, feeding instars.