On latches in biological systems: a comparative morphological and functional study of the retinaculum and the dens lock in Collembola.

BACKGROUND: Springtails have the ability to jump using morphological structures consisting of a catapult, the furca, and a latching system constructed with interaction of the retinaculum and the dens lock. The retinaculum engages in the furca at the dens lock in order to form a spring mechanism. They exhibit diversified morphological traits that serve as adaptations to a variety of terrestrial strata and aquatic surface environments. This comparative morphofunctional study centered on the retinaculum and the furcular region of the dens lock aims to describe the morphological variation between taxa and provide insights into the functional dynamics of the latching mechanism at work in the jumping apparatus. Using SEM, µCT and cLSM, we compared representatives of Collembola taxa, Poduromorpha (Neanura muscorum and Podura aquatica), Symphypleona (Dicyrtomina ornata) and Neelipleona (Megalothorax minimus), and examined extracts of the environment in which they were collected. RESULTS: A retinaculum is absent in N. muscorum, although vestigial muscles were found. Abdominal musculature varies significantly, being more abundant in springtails with clear segmentation (N. muscorum and P. aquatica), and reduced in springtails with fused segmentation (D. ornata and M. minimus). The M.a-ret varies as regards architecture and point of connection with the ramus, which is lateral in P. aquatica and median in the other species studied. The number of teeth in the retinaculum ramus also varies between three in M. minimus and four in the other species. The dens lock of all species studied has two locks and two furrows. CONCLUSIONS: The retinaculum and dens lock interact in a key-lock relationship. The latching and unlatching mechanism from the retinaculum and dens lock appear to be similar in all the taxa examined, occurring by muscle force. This leads us to question the hypothesis that hemolymph pressure may be a force generator in jumping. We offer a reconstruction of the ground pattern of the retinaculum and dens lock and, in addition, an explanation of their functioning and the interaction between them. Finally, we frame the interaction between the retinaculum and the dens lock as a latch in a biological system, a mechanism which functions by force of physical contact.

A morphofunctional study of the jumping apparatus in globular springtails.

Springtails are notable for their jumping apparatus and latch-mediated spring mechanism. The challenge, in the light of the tiny size and rapid movement of these organisms, has been to understand the morphological intricacies of this spring system. This study takes an approach that integrates SEM, MicroCT, cLSM and high-speed video recordings to understand the composition and functionality of the jumping apparatus in Megalothorax minimus (Neelipleona), Dicyrtomina ornata and Dicyrtomina minuta (Symphypleona). We focus on reconstructing, describing, and understanding the functioning of structures such as basal plates, musculature and furca. The dimensions of the jumping apparatus in Dicyrtomina and Megalothorax differ significantly from those in elongated springtails. A hypothesis of functional coherence between taxa, based on muscle connections and basal plates, is postulated. High-speed video recordings provide information on: 1) furca release timing and function during jumping and self-righting; 2) performance properties of manubrium, dens and mucro in interaction with the ground and in take-off; 3) possible pre-release furca moves. The study underscores the need for further research employing a variety of visualization methods in order to explore additional aspects such as retinaculum unlatching and furca flexion/extension muscles.

Four new species of Plumachaetas Salmon (Collembola: Entomobryoidea) from Oceania with comments on the genus and a key to species.

Plumachaetas Salmon is a genus of Entomobryoidea with 11 valid species, all from Oceania. The genus is characterised by the presence of lanceolate scales, the subapical chaeta of the maxillary outer lobe thick and blunt, dental spines absent and a short mucro. In this paper we describe four new species of Plumachaetas from Oceania: P. belae sp. nov., P. loriensis sp. nov., P. mamuensis sp. nov. and P. wanangensis sp. nov. Plumachaetas belae sp. nov. and P. mamuensis sp. nov. are similar to each other, and to P. queenslandica (Schött) and P. yoshii Zhang Deharveng, but are unique in the combination of dorsal and labial chaetotaxy. Plumachaetas loriensis sp. nov. resembles P. halmaherae (Yoshii Suhardjono) and P. sunae Zhang Deharveng but differs in labial, dorsal chaetotaxy and unguiculus shape. Plumachaetas wanangensis sp. nov. is similar to other taxa bearing a mucro with four teeth, but presents a unique combination of macrochaetae on thoracic and abdominal segments. All species are arboreal and were collected in Malaise traps set in humid rainforests. A map of the geographic distribution and ecological remarks on the genus are provided and a key to species with a table summarising the main morphological features of all described taxa in the genus.