Arachnids

Arachnids Definition

Let's begin by stating the definition of arachnids.

Arthropods are invertebrate Animals with exoskeletons, multiple joints, and segmented bodies. At the same time, terrestrial means living on land. In biology, segmentation means splitting the living organism's body into repeated units, as shown in Figure 1. For example, vertebrates such as humans have a segmented vertebral column.

Figure 1: Segments of an arthropod illustrated. Wikimedia, Maxwell Lefroy (Public Domain).

Exoskeletons are the outer hard shell or skeletons consisting of chitin, a structural carbohydrate that supports and protects the arachnid's body.

Around 100,000 species of Arachnids in the world and about 8,000 of them are found in North America. Arthropods are believed to be one of the earliest known terrestrial animals. Arachnids are located under the arthropod phylum Arthropoda and the class Arachnida.

Scientists have found little change between the fossils and the current body plans of the arachnids. Most arachnid fossils come from amber inclusions or fossilized resin from tree sap or gum.

Arachnids Characteristics

Arachnids share many common characteristics. Arachnids have four pairs or eight legs combined, as shown in Figure 2. They also have segmented bodies consisting of a cephalothorax and an abdomen.

Arachnids don't have wings or antennae. They have exoskeletons, are mainly carnivorous, and can only eat liquid foods. Arachnids also cover their prey with digestive chemicals that "melt" them. Afterward, arachnids can suck on their target to consume them.

Arachnids have characteristics that help them survive on land. They have internal respiratory systems such as a trachea and a book lung. These structures allow for gas exchanges internally and externally.

They have modified and jointed appendages: waxy layers covering their cuticle to reduce Water Loss and sensory organs. Some of these arachnids can be poisonous, and some can be parasitic.

Arachnids have simple eyes or ocelli. Simple eyes have a single lens with no complex retina, unlike vertebrates. They are part of the Chelicerata subphylum with six pairs of appendages.

Arachnids lay eggs except for the scorpion, which gives birth to live young. They generally have a three-part life cycle: egg, immature, and mature.

Parts of Arachnids

The body parts of arachnids are split into the cephalothorax and abdomen.

• The cephalothorax comprises the head, thorax, or anterior of the arachnid and is shown in Figure 2.

• The abdomen is the posterior or the parts behind the thorax where the organs are contained in an arachnid and is shown in Figure 3.

The cephalothorax and abdomen together are called tagmata. They have chelicerae or mouthparts that have a fang-like appearance allowing arachnids to eat and protect themselves.

Another necessary appendage attached to the cephalothorax is the pedipalps which work differently in different arachnids. For example, pedipalps help spiders sense things and work as pincers in scorpions. We can think of the pedipalps as "antenna-like."

The four legs, chelicerae, and pedipalps result in six appendages mentioned in the Arachnid characteristics section.

Figure 2: Basic parts of arachnids: spider illustrated. Wikimedia. Public Health Image Library (PHIL), CDC.

Arachnids Examples

After understanding what arachnids are, their characteristics, and general body parts, we can now go over examples of arachnids.

Some of the most common examples of class Arachnida include scorpions, spiders, and ticks, as illustrated in Figure 3. Below we'll go over them in more detail.

Scorpions

Scorpions are part of the order Scorpiones. They are known for their segmented tail with a poisonous stinger in the back and pincers in the front.

• Scorpions mainly live in dry environments like deserts and are nocturnal.

• They are around 2.5-8.3 inches, making them more enormous than most arthropods.

Male scorpions are usually thinner and have longer tails than females. Yellow or light-brown ones can be found in deserts, while those in other habitats are typically brown or black.

• Scorpions breed during warm seasons. The male locates the female through pheromones that the female releases from its abdomen.

There are around 2,000 scorpion species, but only approximately 40 have strong enough poison to kill. On top of this, most toxins are specific to their designated prey. Since scorpions live primarily in harsh deserts, they can slow their metabolism when food is scarce. Ticks have a separate cephalothorax and abdomen, as illustrated by Figure 4.

Spiders

Spiders are part of the order Araneae. Although all spiders have poison, only the brown recluse and the black widow are harmful.

Most spiders eat insects, helping control the pest population and saving crops keeping our food supply safe.

• Contrary to popular belief, not all spiders make webs. Instead, they produce silk to climb, nests, trap prey, etc.

Some spiders have six eyes, although most spiders have eight eyes. The most massive spider is the goliath bird eater tarantula, and it's around 4.75 inches (ca. 12 cm).

Most spiders are threatened by habitat loss due to anthropogenic or human impact. They live in diverse habitats, from rainforests to gardens.

Ticks

Ticks are part of the order Acari. There are around 900 tick species found in the world.

Ticks are parasitic as they feed on the Blood of their host organism. They find their host through "questing." Whereby they perch and wait on top of the tips of vegetation. Once a good host appears, the ticks will grab onto them. Ticks know when hosts walk by as they respond to specific stimuli: thermal heat from the body, vibrations, etc.

Once on a host, the female tick usually feeds for days, while the male tick only takes Blood meals once in a while and stays on the host for more extended periods to look for a mate.

A tick's habitat differs based on the type of host they prefer. But mostly, they need humid environments because they are prone to desiccation or drying out. This is why they are primarily found in areas with trees and other plants.

Although all arachnids have the same parts, ticks have a fused cephalothorax and abdomen, unlike scorpions, pictured in Figure 4.

Arachnids vs. Insects

Most people tend to think that arachnids are the closest relatives to insects. But this is not true, as that title belongs to the horseshoe crabs.

Insects and arachnids are both parts of the phylum Arthropoda. However, insects belong to the class Insecta while arachnids belong to Arachnida.

As discussed earlier, arachnids have two body segments: the cephalothorax and the abdomen; refer to Figure 2. In comparison, insects have three body segments: a head, a thorax, and an abdomen, as illustrated in Figure 5.

Insects have an antenna, while arachnids do not. However, arachnids do have antennae-like structures called pedipalps.

Insects also have three pairs of legs or six legs combined. In contrast, arachnids have eight legs or four pairs of legs.

Arachnids have a three-part life cycle, while insects have four. In addition, plenty of insects undergo metamorphosis, like the monarch butterfly.

Since both organisms are arthropods, they both have exoskeletons and are invertebrates. There are around 1 million insects reported making them about 80% of all animal species on our planet! Insects, like arachnids, have essential roles in the ecosystem.

For instance, insects act as pollinators for many flowering plants and are food sources for animals such as arachnids. Simultaneously, arachnids benefit gardens and crops by eating the insects that tend to devastate them.

Figure 5: Insect body parts illustrated. Wikimedia, Pearson Scott Foresman (Public Domain).