In the more primitive insect groups, males synthesize specialized sperm carriers called spermatophores to protect the sperm during their transfer to the female genital tract. Insects are believed to have descended from aquatic ancestors whose males released their sperm directly into the water. In adaptation to a terrestrial existence, however, a more precise method of mating was required to prevent the sperm from desiccating. The spermatophore thus represents an initial adaptation for life on land that protects the male gametes until they are within the female reproductive tract. In more advanced insects, the sperm are transferred directly in seminal fluid and the spermatophore is no longer produced.
SPERMATOPHORES IN APTERYGOTE INSECTS
The insect spermatophore is synthesized by the male accessory glands. It consists of a viscous secretion that is shaped by the internal structures of either the male or the female once it has been inserted into the female reproductive tract. A change in pH may account for the transition from a liquid secretion by the male accessory glands to a more solid, gel-like mass. The sperm are contained within the saclike ampulla of the spermatophore before it is transferred to the female.
The structure of the spermatophore and the mechanism of transfer differ among the insect orders. In many primitive aptery-gote insects, such as the collembolans, the spermatophore consists of a drop of sperm placed in a simple sac at the end of a stalk on the ground. Females must find the spermatophore themselves and actively take it up into the genital tract. The behavior is more complex in other species of Collembola, where the male may actively manipulate the female and direct her to the spermatophore he has already deposited. These males have not developed the modifications necessary to grasp the female for conventional mating. Similarly, some Thysanura males deposit an unstalked spermatophore on the ground, but place a wall of silken threads around it to signal to the female that the spermatophore is nearby. Because all these insects inhabit humid soil environments, there is little chance for the sperm to dry out before the females find them and take them up.
SPERMATOPHORES IN PTERYGOTE INSECTS
In pterygote insects that produce a spermatophore, transfer to the female is more direct. Odonate males deposit their spermatophore from their genital opening at the tip of the abdomen to the secondary genitalia on the anterior segments of the abdomen. During copulation, the male grasps the female with the tip of his abdomen while her abdomen loops forward to receive the sperm from his secondary genitalia. In many orthopterans, the spermatophore is inserted into the female genital tract with a long spermatophore bulb that protrudes from the female genitalia. The female may eat the exposed bulb once the sperm have left, deriving the considerable nutritional investment that the male has made. Some males present a nuptial gift during and shortly after copulation to distract the female and prevent her from eating the spermatophore before the sperm have left it. In the cricket Acheta domesticus, the physical coupling of the male and female is followed by the insertion of the spermatophore into the female genital tract and the attachment of its long tube to the female’s ovipositor. The sperm travel into the spermatheca of the female, and after 30-40 min, the flasklike spermatophore is dislodged.
In the more advanced pterygotes, the males form the spermato-phore in a more recently evolved structure in the female genital tract, the bursa copulatrix, and the sperm then move to the spermatheca(e) of the female for ultimate storage and utilization. In some Lepidoptera, the bursa contains spines that rupture the spermato-phore and allow the sperm to escape. Once the sperm have escaped, the spermatophore may be digested by enzymes secreted into the bursa, and its raw materials exploited by the female for egg production. Male Anopheles mosquitoes produce a modified spermatophore that serves as a mating plug to temporarily block the genital tract and prevent the female from mating with other males.
The metabolic costs of synthesizing a spermatophore may be one reason for its ultimate disappearance in many of the higher insect orders. Accompanying the loss of the spermatophore has often been the development of a male intromittent organ that is capable of placing the sperm directly into the bursa copulatrix or the spermatheca(e), making the spermatophore unnecessary.
