Abstract
Many insects possess a hyper-elongated intromittent organ with a diameter of only a few micrometers. Using morphological and theoretical approaches, we investigated the biomechanics of sperm transfer through such organs by calculating (1) how far and how fast sperm could fill in the penis by capillary action, (2) how much capillary pressure is generated in the penis, and (3) how much pressure is needed to pump sperm out of the penis. The results enabled us to propose the following hypotheses: (1) penile filling basically occurs by capillary action, and (2) sperm transport to females occurs by contracting the sperm pump muscles or by active propulsion of spermatozoa. Potential experimental approaches to test these hypotheses are discussed.
Highlights
Transferring fluids through a very slender duct does not seem to be an easy task for small animals
While the male reproductive system of Lema coronata does not feature a sperm pump, those of all other studied species are of type A and include such a sperm pump (Fig. 3), which is a swollen part of the ejaculatory duct and is situated at the basal area of the duct
A sclerotized and strongly melanized sclerite was observed at the proximal part of the sperm pump in Cassida rubiginosa and C. vibex, and a less developed pump was found in Oomorphoides cupreatus, but not in Zeugophora annulata (Fig. 3, arrow heads)
Summary
Transferring fluids through a very slender duct does not seem to be an easy task for small animals. Fluid transfer through a slender tube occurs during copulation in many species of a wide range of taxa with a hyper-elongated intromittent organ[15,16]. In the extreme case of the leaf beetle species Lema coronata, the intromittent organ reaches a length of approximately 10 mm with a diameter of only 2 μm[17]. These structures are solely used as sperm transferring organs[18,19,20,21,22,23], with one exception of a zorapteran species, whose elongated intromittent organ is plausibly used only for sperm displacement[15]. Based on our results and literature data on analogous fluid drinking mechanisms of insects, we propose possible sperm transfer mechanisms for the studied species and suggest relevant future experiments
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