Aphagy and vestigial stylets in first-instar nymphs of Aradidae (Hemiptera, Heteroptera). (January 2023) - Rakitov, Roman
Abstract: 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. Highlights: Stylets of 1 st -instar nymphs of Aradidae are short, non-functional—the first report of vestigial mouthparts in Heteroptera. Other mouthparts, the labium, food canal, and associated muscles (including stylet muscles) are normally developed. Non-feeding 1 st -instar nymphs with functional mouthparts have been known in many other families of Pentatomomorpha. A new hypothesis links this phenomenon with a special role of the first instar in acquisition of microbial gut symbionts. A presumed loss of symbionts in the ancestors of Aradidae may have led to reduction of stylets in their 1 st -instar nymphs.