Although most arthropods reproduce through direct sexual reproduction and lay eggs, there is much more diversity beyond that.
We will look at several reproductive features and methods, and review some examples of species that use each type.
What Are Arthropods?
When you think of mosquitoes, crabs, or spiders, you most likely picture animals that are completely different, correct? What if I told you that these animals are in fact related? All three of these belong to the phylum Arthropoda and are called arthropods. Imagining the above animals in your head, you should be able to envision their segmented legs, segmented bodies, and their exoskeleton, the skeleton outside of their body. These are some of the basic features that separate arthropods from other phyla of animals.Arthropods have a major body cavity that is split into compartments and filled with blood. It is called the hemocoel.
In arthropods, these blood-filled compartments keep their organs constantly bathed in blood, helping them survive. It is in one of these blood-filled compartments where the reproductive system exists.
The reproductive system of arthropods is found on the ventral or belly side of the animal.
Each species of arthropod has its reproductive system positioned in a slightly different place according to its body plan, but all will connect to openings on the ventral side of the animal. Most arthropods are gender specific and have male or female gonads, or sex organs. In some species, such as in barnacles, they are hermaphroditic, that is possessing both male and female sex organs.In some arthropods, sperm cells are covered by a protective coating called a spermatophore. This helps keep the sperm cell safe from anything it might encounter while it waits to fertilize an egg. This is important to terrestrial arthropods, because it guards the sperm from drying out if it has been deposited on the ground.
It’s equally important to aquatic arthropods, so that the water doesn’t mix with the sperm. In some cases, a spermatophore is not necessary and sperm swim into the female during copulation, like in human reproduction.
Arthropods have a wide range of reproduction methods. Typical sexual reproduction, where a male of the species fertilizes the eggs of the female, can be done in a few different ways.Some arthropods reproduce using the deposition method.
This is when the male deposits spermatophore on the ground attached to a stalk that keeps it stuck there. Specific pheromones attract the female, who will then pick it up and place it inside of herself, causing fertilization to occur. Centipedes, millipedes, spiders, scorpions, and ticks often reproduce this way.Rarely, arthropods do external fertilization, which is when the female releases the egg and the male releases the spermatophore or sperm at the same time to fertilize the eggs outside of the body. This occurs in horseshoe crabs.Many arthropods, such as insects, reproduce through impregnation, where the male inserts the spermatophore or sperm directly.When female eggs develop without being fertilized by male sperm, we refer to this type of reproduction as parthenogenesis.
You can think of this as a virgin birth. Ants, bees, and wasps all practice a form of parthenogenesis.
Eggs vs. Live Birth
If the young are born live, it is referred to as a viviparous birth. This is not common, but scorpions are an example of this. As the young are born, they climb onto their mother’s back. However, most arthropods lay eggs, which then hatch when they are ready.
These include the majority of insects (flies, butterflies, dragonflies, beetles, crickets, bees, etc), as well as millipedes, centipedes, and spiders.
Arthropods include quite the variety of animals, but they all have exoskeletons and feature a body cavity called a hemocoel, which is divided into small sections and saturates the body organs with blood. Underneath the arthropod on the ventral (belly) side there are small holes that attach to ducts that are attached to the reproductive organs.The particular species of arthropod determines how exactly they will reproduce, but there are several ways:
- Deposition, where the male deposits sperm on the ground and the female picks it up and deposits it inside herself.
- External fertilization, in which sperm and egg are released at the same time for fertilization outside the body.
- Impregnation, in which the male inserts the spermatophore or sperm directly into the female.
- Parthenogenesis, or virgin birth, in which eggs develop without any sperm.
In deposition, external fertilization, and impregnation, the sperm is often encased in a spermatophore, which protects the sperm from any outside interference.
But free sperm may also be transferred. Though some arthropods, like mites and scorpions, give live birth, the majority lay eggs that later hatch. As you can see, arthropod reproduction is incredibly diverse.