Detachable setae are true setae that can be easily removed with any kind of mechanical stimulation and that evolved as a defensive structure against predation in at least four groups of Arthropoda. The class is subdivided in two main morpho-ecological groups: urticating hairs of Lepidoptera (Nodotontidae: Thaumetopoeinae, Erebidae, Saturniidae, Zygenidae) and the spider family Theraphosidae (subfamily Theraphosinae), and anchor-like setae of Polyxenidae millipedes (Myriapoda: Polyxenida) and larvae of Dermestidae (Insecta: Coleoptera).
Hastisetae are a specific group of detachable setae characterizing the larvae of Megatominae (Coleoptera: Dermestidae). These setae are located on both thoracic and abdominal tergites and they are the primary defence of the larva against invertebrate predators.
Hastisetae, single or aggregate, function as an extremely efficient mechanical trap, based on an entangling mechanism of cuticular structures (spines and hairs) and body appendages (antennae, legs and mouthparts). It is believed that this defensive system evolved primarily to contrast predation by invertebrates, however it has been observed that hastisetae may affect vertebrates as well. Although information on the impacts of vertebrate predators of the beetles is limited, hastisetae have been shown to be a possible threat for human health as an important contaminant of stored products (food and fabric), work and living environment; in particular, the exposure to hastisetae seems to cause allergic reactions and the insurgence of skin rashes, asthma, conjunctivitis, and digestive system inflammation.
A review of past and recent literature on dermestid larvae has revealed that despite these structures are vaguely indicated as one of the distinctive characters in species identification, very little is known about their ultrastructure, evolution, and mechanism of action. Furthermore, despite their apparent medical interest almost nothing is known about their harmfulness in comparison to urticating hairs of lepidoptera and tarantulas.
The present work provides the state of knowledge on hastisetae in Dermestidae and develops a research line intended to bridge the existing knowledge gaps.
The first part of the research is dedicated to the investigation of the fine morphology of the external microstructure of the hastisetae; the insertion on integument, the pedicel, the shaft, and the apical head are described and illustrated in detail, and the first observations of active defensive behavior based on hastisetae are recorded and presented.
The second part is devoted to the ultrastructure investigation of the hastiseta with a major focus on the socket and subcuticular structure with the intent to investigate and shed a light on a possible secretory function associated with hastisetae.
Possible implications to the systematics of skin beetles are proposed based on the results of the study. Furthermore, a new morphological and functional interpretation of the hastiseta and the associated cellular is provided, suggesting possible future research developments