Spermatophore

Spermatophore (from the Greek sperma, "seed," and phoros, "bearing") is a structured capsule, sac, or gelatinous packet containing spermatozoa and accessory seminal secretions, synthesized by the male accessory reproductive glands (MAGs) and transferred as a single unit to the female reproductive tract during mating. Originally evolved as a key terrestrial adaptation to prevent sperm desiccation when early Paleozoic arthropods (arachnids, myriapods, springtails, and bristletails) transitioned from aquatic external fertilization onto dry land without intromittent copulatory organs, spermatophores remain widespread across insects (including Orthoptera, Lepidoptera, Coleoptera, Blattodea, and Diptera), cephalopod mollusks, decapod crustaceans, and caudate amphibians (salamanders and newts). Beyond functioning as a mechanical delivery vehicle for sperm, the spermatophore in many insects has been evolutionarily elaborated by sexual selection into a large edible spermatophylax ("nuptial gift")—accounting for up to 15% to 40% of the male's total body mass—that provisions the female's developing oocytes, modulates her post-mating neuroendocrine receptivity via seminal fluid proteins, and governs cryptic female choice during sperm competition.

Comparative arthropod reproductive morphology traces three evolutionary stages in the deployment of spermatophores (Ghilarov, 1958; Proctor, 1998):

1. Non-Contact Indirect Spermatophore Transfer (Collembola, Diplura, and Soil Mites): Male springtails (Collembola) and campodeid diplurans deposit tiny, stalked proteinaceous spermatophores onto damp leaf litter or soil substrates even in the absence of a female, surrounding them with volatile aggregation pheromones; wandering females locate the droplet chemically and take it up through their genital pore.
2. Paired Courtship-Guided Substrate Transfer (Scorpions, Pseudoscorpions, Amblypygi, and Urodela): Male scorpions (Scorpiones), whip spiders (Amblypygi), silverfish (Zygentoma), and plethodontid/ambystomatid salamanders cement a complex, lever-actuated spermatophore onto the ground and grasp the female in a stereotyped promenade à deux ("walk for two"), maneuvering her body directly over the stalk until mechanical triggering of the cap (operculum) everts the sperm mass into her gonopore.
3. Direct Intromittent Internal Transfer (Pterygote Insects): In winged insects (Pterygota), the male aedeagus inserts the spermatophore directly into the female's bursa copulatrix (or externally onto her genital opening, as in crickets and katydids), where the capsule either osmotically evacuates spermatozoa into the female's spermatheca via a hollow ejaculatory duct or is digested by female bursal proteases.

In ensiferan Orthoptera—most famously katydids (Tettigoniidae), camel crickets (Rhaphidophoridae), and Haglidae (Cyphoderris)—the male's accessory gland complex secretes a bipartite spermatophore divided into two functionally distinct compartments:

• The Sperm Ampulla: A tough, multi-layered proteinaceous flask containing the haploid spermatozoa alongside a specialized osmotic pressure body (evacuation body) and prostaglandin-synthesizing enzymes. The narrow neck of the ampulla is threaded directly into the female's genital chamber at the close of copulation.
• The Spermatophylax (Edible Nuptial Gift): Surrounding or attached to the exterior of the ampulla is a large, bilobed, translucent gelatinous mass termed the spermatophylax, composed of water (~80–85%), specialized storage glycoproteins (spermatophylaxins rich in proline, glycine, and glutamic acid), free amino acids, and phospholipids—but completely devoid of sperm.

The Nuptial Gift vs. Ejaculate Protection Hypothesis

Why do male bush-crickets (Requena verticalis, Poecilimon veluchianus, Ephippiger ephippiger) invest up to 25% to 40% of their entire body weight into a single spermatophylax, a physiological cost so high that males lose their ability to stridulate for several days after mating and roles in sexual selection can reverse (with choosy males rejecting small females)? Two complementary evolutionary hypotheses explain the size of the spermatophylax (Darryl T. Gwynne, 1984; Karim Vahed, 1998):

1. The Ejaculate Protection ("Mating Effort") Hypothesis: Immediately after uncoupling from the male, the female katydid bends her head beneath her abdomen and begins consuming the external spermatophylax. Meanwhile, sperm are slowly pumped out of the attached ampulla into her spermathecal duct over a period of 45 to 180 minutes. If the male produces a small spermatophylax that the female finishes eating in only 20 minutes, she immediately eats the sperm-containing ampulla next—terminating sperm transfer prematurely and drastically reducing the male's paternity! Across > 30 species of Tettigoniidae, comparative phylogenetics confirms that spermatophylax consumption time correlates linearly with the time required for complete ampulla evacuation.
2. The Paternal Investment ("Nutrient Provisioning") Hypothesis: Radiotracer studies using ¹⁴C- and ¹⁵N-labeled amino acids injected into males demonstrate that proteins from the ingested spermatophylax are absorbed across the female's midgut and incorporated directly into the vitellogenin yolk of her developing eggs within 48 hours, increasing both egg mass and offspring starvation survival when ambient food is scarce.

In many insects, the spermatophore is also a chemical weapon and a neuroendocrine switch:

• Defensive Alkaloid and Terpenoid Provisioning: Male rattlebox moths (Utetheisa ornatrix) sequester toxic pyrrolizidine alkaloids (PAs) from Crotalaria host plants as caterpillars and pack up to 30% of their systemic PA stores into their spermatophore. During courtship, the male broadcasts a pheromone (hydroxydanaidal) synthesized directly from his PA reserves, allowing the female to gauge the chemical potency of his spermatophore before mating; after copulation, she allocates the male's PAs into her eggs to render them unpalatable to coccinellid beetles and chrysopid lacewing larvae (Thomas Eisner & Jerrold Meinwald, 1995).
• Bursal Stretch Receptors and Anti-Aphrodisiac Pheromones: In Pieris rapae and Heliconius butterflies, the physical volume of the spermatophore distends mechanosensory stretch receptors in the wall of the female's bursa copulatrix, sending ascending inhibitory signals through the ventral nerve cord that immediately shut down female sex-pheromone calling and trigger mate-rejection postures. Simultaneously, male Heliconius butterflies lace the exterior of their spermatophore with volatile anti-aphrodisiac pheromones (such as β-ocimene or methyl salicylate) that odorize the female and repel rival males—triggering an evolutionary tug-of-war in which females evolve spiked bursal signa (sclerotized internal teeth) to grind and digest the spermatophore faster so they can remate sooner.

1. Gwynne, D. T. (1984). Courtship feeding increases female reproductive success in bushcrickets. Nature, 307(5949), 361–363. https://doi.org/10.1038/307361a0
2. Eisner, T., & Meinwald, J. (1995). The chemistry of sexual selection. Proceedings of the National Academy of Sciences, 92(1), 50–55. https://doi.org/10.1073/pnas.92.1.50
3. Vahed, K. (1998). The function of nuptial feeding in insects: A review of empirical studies. Biological Reviews, 73(1), 43–78. https://doi.org/10.1111/j.1469-185X.1997.tb00025.x
4. Gabrieli, P., Kakani, E. G., Mitchell, S. N., Mameli, E., Want, E. J., Mariezcurrena Anton, A., ... & Catteruccia, F. (2014). Sexquitoes: Transfer of male steroid hormone 20-hydroxyecdysone in the Anopheles gambiae mating plug. Proceedings of the National Academy of Sciences, 111(46), 16353–16358. https://doi.org/10.1073/pnas.1410488111