The formula derived by Nobuyasu Sato (1980 J. Phys. D: Appl. Phys. 13 L3-L6), for the current flowing in an external circuit due to the motion of charged particles in a gap is extended to include a time-dependent applied voltage. The effects of negative ions, and electron, positive ion and negative ion diffusion are also included. It is found that Sato's original equation still describes the contribution of charged particle motion to the external circuit current. The total circuit current is determined by adding the displacement current that effectively flows across the gap to the current determined by Sato's equation. Some comments are also made about the interpretation of Sato's equation in three dimensions.

A formula for discharge current flowing in a space charge filled gap is derived for a general geometry of electrodes from the energy balance equation in which the displacement current caused by the motion of the charged particles in the gap is taken into account. It is found that in the formula obtained the influence of the field component produced by the space charge does not appear explicitly and the formula is compatible with the principle of superposition.

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In polyandrous mating systems, sperm competition and cryptic female choice (CFC) are well recognized as postcopulatory evolutionary forces. However, it remains challenging to separate CFC from sperm competition and to estimate how much CFC influences insemination success because those processes usually occur inside the female's body. The Japanese pygmy squid, Idiosepius paradoxus, is an ideal species in which to separate CFC from sperm competition because sperm transfer by the male and sperm displacement by the female can be observed directly at an external location on the female's body. Here, we counted the number of spermatangia transferred to, removed from, and remaining on the female body during single copulation episodes. We measured behavioral and morphological characteristics of the male, such as duration of copulation and body size. Although males with larger body size and longer copulation time were capable of transferring larger amounts of sperm, females preferentially eliminated sperm from males with larger body size and shorter copulation time by spermatangia removal; thus, CFC could attenuate sperm precedence by larger males, whereas it reinforces sperm precedence by males with longer copulation time. Genetic paternity analysis revealed that fertilisation success for each male was correlated with remaining sperm volume that is adjusted by females after copulation.

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1In the Japanese pygmy squid Idiosepius paradoxus, females often pick up 2 the spermatangium using their mouth (buccal mass) after copulation. To examine

To examine the mechanism of sperm storage in Idiosepius paradoxus, here we describe aspects of the mating behavior of I. paradoxus and the morphology of the spermatozoa and the seminal receptacle after copulation. The seminal receptacle is located in the ventral portion of the buccal membrane surrounding the buccal mass, and opens inside the buccal membrane. It branches into approximately six sacs, similar in appearance to a bunch of bananas, and its wall consists of cuboidal ciliated epithelial cells (with oval nuclei) surrounded by a connective tissue. Multiple vacuoles are distributed in the bottom region of each sac. These histological and morphological characteristics differ from previous reports for loliginid squids and cuttlefish. In all except one receptacle observed in this study, sperm were stored near the bottom of each sac, and each sperm was facing the sac bottom. We observed spermatozoa in the entrance of the seminal receptacle in only one squid. These results suggest that spermatozoa were actively moving, and that sperm actively swam to the seminal receptacle. The volume of sperm in the seminal receptacles of the squid that had copulated eight times was the same as that in the squid that had copulated 29 times, which suggests that the seminal receptacle was filled after approximately eight copulation events. A squid that had copulated nine times retained a significant number of sperm in the seminal receptacle after spawning, suggesting that all of the sperm in the receptacle was not depleted after one spawning event.