The Reproductive Cycle of the Black Widow Spider

Term Page by John Chmielewski

As many biology students in high school and especially college have learned to their morbid delight, insect behavior in relation to the reproductive process can be very strange and difficult to comprehend. The classic example would be spiders. Spiders engage in a wide variety of rituals before engaging in the sexual process. The Black Widow spider, whose name we have all come to fear, is a very interesting spider to observe. Observations of the various stages of the reproductive process are especially interesting. The reproductive process has been divided into three main stages. Courting between the male and female is the first act that must occur. Once that stage is successful it leads towards sexual acts and insemination. Finally post-sexual acts shall be described. The courting style between the female and the male shall be discussed first. The Black Widow spider (Latrodactus mactans) (Lanham 1964) is one type of spider whose reproductive cycle is very interesting. Before an explanation and description are given as to its reproductive cycle, one must first be able to recognize a Black Widow through various characteristics. The female can be recognized by its shiny black body and by a distinctive red hour glass figure on the underside of its body. This red area is also sometimes to be found as two separate spots of red. The females of the species range in size from 5.0 to 13.5 millimeters (Faith 1998). The males are completely black and range in size from 3.0 to 5.0 millimeters (Faith 1998). The male black widow spider is rarely seen and this topic shall be discussed later in this paper. The activities that lead up to the reproductive process are the most important in the life cycle of the Black Widow spider. The main objective of any species would be to procreate the species and this makes the courting process essential, especially in the case of the Black Widow spider. When the male and female meet, there is often some type of conflict. The male in this case may be ambivalent; the desire to flee and the desire to proceed are the only two reactions that it may be considering (Waldbauer 1996). The use of pheromones in spiders courting has been assumed. Several biological studies have proven their existence and still none have been identified in the courting ritual of the Black Widow (Schulz and Toft 1993). So it can be assumed, for the time being, that pheromones do not play a part in the courting ritual of the Black Widow spider. The male has to approach the female spider in a careful way. There are certain ways to accomplish this if any sexual acts are going to occur. When the male attempts to climb onto the female’s web, a large amount of caution must be taken. A female spider who feels her web twitch and vibrate hurls herself very quickly to that point and injects venom into the prey so that it becomes paralyzed and cannot escape. The courting male does not want to be mistaken for prey so the approach and climb onto the web must be done very delicately. In order to avoid being mistaken for a meal, the male sends a type of telegraphic message to the female by plucking on specific strand in her web that he hopes she will understand (Waldbauer 1996). Plucking on a strand of her web is done to ensure that the female knows he is a suitor, not a meal. Once the male accomplishes this first step he proceeds to move further onto the web and begins to approach the female. This must be done very slowly and sometimes can last for many hours, even days. Once the male is close enough to the female he begins to gently stroke the female with his pedipalps to try to discriminate her from having any changes of mind to eat the male. Once the female has decided not to eat the male, then the process of copulation can take place. The process of copulation is unique in spiders especially the Black Widow. The male spider has a unique and strange way of delivering the semen into the genital opening of the female. The genital opening of both the male and the female are located on the abdomen. The males copulatory organs are not located on his abdomen. They are complicated structures which are located on his pedipalps. Pedipalps are the long leg-like structures which are usually used as a type of sensory organ. However, in the process of reproduction, they have specific functions, which are used as aids in the insemination process. The male cannot immediately inseminate the female. The structure of the male’s reproductive organs does not allow this. The male spider must first spin a small "sperm web" and then deposit a small amount of semen onto it. Before this can happen though, the spermatozoa must be produced. Spermatozoa are the haploid reproductive cells that the male spider produces. The spermatozoa are produced in the male tubular testes that are located on the floor of the abdomen (Snow 1970). There are tubes called the convoluted vasa diferentia that lead from the testes to the genital opening in the center of the epigastic furrow (Snow 1970). The furrow is located in the abdomen region of the male spider. The spermatozoa are produced and transported through the tubes to the furrow. The male can now produce and amount of semen and place it on the "sperm web" that he has already spun. The male now must have some way to inject the female with this semen or reproduction of the species would not be possible. This is accomplished with the help of the modified pedipalps. The male palp is attached near the end of the pedipalps and in its resting state is deflated. When the process of copulation is about to take place, the palp inflates and the bulb becomes exposed. Structures that makeup the male palp are the tarsus, which the bulb is attached to, the bulb itself, the spermophore, and the embolus. The embolus is the opening that is inserted into the female opening. The spermophore is a tubule inside of the bulb through which the semen is transported. This bulb acts like an eye-dropper, first sucking up a drop of semen and then squeezing it into the female genital opening. It is not yet certain how this is exactly done, since it does not seem that blood pressure is involved (Wilson 1971). One theory is that it is pushed out of the embolus by secretions produced at the blind end of the spermophore. Blood pressure is, however, used to inflate the bulb and bring it into use (Wilson 1971). Once the male has absorbed the semen he is ready for copulation to occur. The female allows the male to approach her and the male then inserts his papal organs, specifically the embolus, into the female’s copulatory duct that leads to the spermathecae. This is the sperm receptacle that can store the spermatozoa that is inserted by the male. Once injected, the spermatozoa are stored there until it is needed to be used. The female’s eggs are produced in the ovary and the eggs then travel up through the oviduct and eventually reach the vagina. Once the eggs reach this upper region of the vagina, the spermatozoa are released from the spermathecae and are allowed to fertilize the eggs. When each of the haploid gametes meet they form the diploid zygote. This zygote then develops through replication of cells and eventually forms into an egg. The number of eggs laid varies from spider to spider but they lay approximately a few hundred eggs. Another unique feature is that even though the female lays several batches of eggs during her lifetime, she only requires a single mating (Wigglesworth 1964). The semen received from a single mating contains enough spermatozoa to fertilize all of her eggs and the spermatozoa can survive for a long amount of time so that they can be used for future fertilizations . Once the eggs are fertilized, the female places the eggs in a cocoon that she has made. The eggs hatch within the cocoon and the spiderlings emerge. After a few days, they moult, begin the stage where they begin to develop eyes, spinnerets, setae, claws, and the ability to feed (Wigglesworth 1964). They can now be called spiders. This leads into the third stage which are post-sexual acts. The phenomenon known as sexual cannibalism occurs in this stage. It is common in spiders that after copulation, the female kills and eats the male. This frequently occurs in Black Widow spiders. That is a main reason why males are very difficult to find in nature. After copulation, if the female has not had a meal, she will usually attempt to kill and eat the male, with the assistance of her venom (Lemonick 1996). That is why this spider gets the name the Black Widow. The only defense for the male is to use his pedipalps for some type of protection from her assault. If the male has been in previous battles with other spiders and has damaged his pedipalps, his chances of surviving are very small. Since the female is usually the larger of the two, the female is able to get her way. If the male is able to leave, he then searches for another female so the reproductive cycle may be allowed to continue. The Black Widow is a very interesting spider, especially in regard to its reproductive process. The courting rituals, copulation, and the post-sexual acts make the Black Widow a unique spider to study. The renewal of this cycle has allowed this species to evolve to the point that it is at today. As evolution continues to occur it shall be interesting to see how this spider shall reproduce in the future. The acquired knowledge of why exactly this process occurs, is one that is beneficial to anyone who has ever been curious about spiders. This paper has hopefully given an explanation of the entire reproductive cycle of the Black Widow spider.

Works Cited

Faith M. Oi. "The Black Widow Spider." The Black Widow. August 1997. (7 April 1998) 1-2

Lanham, Erl. The Insects New York: Columbia University Press, 1964: 14

Lemonick, Michael D. "Sex as Suicide" Time, Jan 15, 1996 v147 n3: 60

Schulz S. And S. Toft "Identification of a sex pheromone from a spider" Science,

June 11, 1993 v260 n5114: 1653

Snow, Keith R. The Arachnids: An Introduction New York: Columbia University

Press, 1970: 32-33 Wilson, Edward O. The Insect Societies Boston: Harvard University Press, 1971: 37

Waldbauer, Gilbert. Insects Through the Seasons London, England: Harvard

University Press, 1996: 49

Wigglesworth, V.B. The Life of Insects Cleveland: The World Publishing Company, 1964: 45,108

johnchem@hotmail.com

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