Arthropods

This page is part of a series introducing various animal phyla. You should look at all these pages:

In this lab you'll be introduced to the phylum Arthropoda; later, there will be another lab with a special emphasis on insects.

Objectives:

After completing this lab, you should be able to explain the following concepts:
Features of the arthropod body plan
Major groups of arthropods

The phylum arthropoda is by far the most diverse of all the animal phyla in terms of number of species, habitats and ways of living. There are at least a million arthropod species, accounting for over 80% of known animal species. Arthropods have adapted to many different environments (marine, freshwater, and terrestrial); few other animal phyla are in such a diverse range of places.

The arthropod body plan

Arthropods have a number of distinctive features in their body plan:

Three tissue layers formed in embryo, like most animals.
True coelom.
Segmented body with specialized and fused segments. Arthropods are clearly segmented, and the different segments are very different from one another in form and function. Also, arthropod bodies are made of several groups of fused segments; the fused segments are called tagmata (singular: tagma), and they act like individual super-segments. Each group of arthropods has a characteristic set of tagmata; for example, insects have three main tagmata, while spiders have only two.
Cuticle. Arthropod bodies are covered with a cuticle made of chitin, forming an exoskeleton have exoskeletons made of chitin (the same complex carbohydrate found in fungal cell walls) and protein. In some arthropods (such as crabs), the exoskeleton is made more rigid with calcium deposition. The first arthropods lived in the ocean; their exoskeletons protected them from attack and provided places for their muscles to attach. When later groups of arthropods moved onto the land, it turned out that the exoskeleton happened to be very functional in preventing the body from drying out. This is a good example of how a characteristic that evolved in one situation can become important in organisms adapting to another situation. Having an exoskeleton also affects a couple of other important aspects of life:
Jointed appendages. Most arthropods have multiple jointed legs, antennae, mouthparts, and other body parts as part of the exoskeleton.
Ecdysis and Metamorphosis. Many animals can simply grow continuously throughout their lives. Arthropods, however, are confined in their relatively rigid cuticle. In order to grow, they must go through ecdysis, or molting, and crawl out of the old exoskeleton. Then they quickly grow bigger by absorbing water before they form a new hard cuticle. Many arthropods go through multiple molts in their life cycle. Some arthopods go through a distinct metamorphosis, in which they change their body form dramatically as they molt and form a new exoskeleton. The classic example of this is the metamorphosis of a caterpillar into a butterfly.
Gas Exchange & Osmoregulation. Confined in their exoskeletons, arthropods need special structures for gas exchange, osmoregulation, and excretion. These specialized structures also seem to create opportunities for some arthropods: with their tracheal system for gas exchange and their Malphigian tubules for osmoregulation, the insects are able to live in dry conditions that would kill most invertebrates.
Circulatory system. Arthropods have open circulatory systems. Like annelid worms, arthropods are strongly segmented, but unlike the annelids, the segmentation does not divide the body into separate compartments. Thus, hemolymph can freely flow from one body segment to another, and a closed circulatory system is not necessary.

Major groups of arthropods

The amazing evolutionary diversification of arthropods has made their phylogeny difficult to figure out, and the experts don't always agree on how to draw the arthropod cladogram. In this lab, we'll just consider a few of the most prominent arthropod groups, and then look at insects in more detail. Following are three major subphyla of arthropods, with some of their defining features.

Subphylum Crustacea: crabs, shrimp

Body Tagmata: Often three (head, thorax, abdomen), but sometimes two (cephalothorax and abdomen, as in crabs).
Legs: the number of legs is variable in crustaceans.
Antennae: Two pairs (other arthropods have one pair or none).
Gas exchange organs: gills. Most crustaceans are adapted to life in the water.

Most crustaceans are aquatic, but one terrestrial crustacean you might see in lab is Armadillidium vulgare, the common pillbug. Also known as a wood louse, this little isopod crustacean lives on land but does not have a waxy waterproofing layer on the outside of its cuticle, so it does not resist desiccation as well as insects do. Pillbugs must live in damp places. Also, their main gas exchange organs are gills, which they must keep wet. For detailed anatomy, see Armadillidium vulgare on Invertebrate Anatomy Online. That page is dense with terminology, but you might want to look for a few features: marsupium (brood pouch), two pairs of antennae, compound eyes, corpus alatum, endopods (gills). You won't be tested on any of this anatomy, but it's interesting to try to find it. See also: Armadillium photos on flickr.

Subphylum Chelicerata, class Arachnida

Chelicerates are named for having chelicerae, which are appendages that resemble fangs or pincers. There are two main classes in this subphylum: Arachnida, which includes spiders, ticks and scorpions; and Myriapoda, which includes centipedes and millipedes. In this lab, we'll only consider arachnids.

Body tagmata: Two main tagmata: head and cephalothorax.
Legs: Four pairs.
Antennae: None.
Gas exchange organs: Spiders have both book lungs and tracheae — two different kinds of organs for gas exchange. For more information on this strange system, see these articles: The respiratory complementarity of spider book lung and tracheal systems and Bimodal breathing in jumping spiders: morphometric partitioning of the lungs and tracheae in Salticus scenicus (Arachnida, Araneae, Salticidae).

Subphylum Hexapoda (including Insecta)

The vast majority of hexapods are insects; there are also some other members of this subphylum, but we'll only consider insects in this lab.

In addition to the typical arthropod features, insects have some distinctive characteristics.

3 main tagmata. Insect bodies are divided into three main parts: head, thorax, and abdomen. The thorax is where all the legs and wings are attached.
Legs: Three pairs (hence the name Hexapoda, meaning "six legs").
Antennae: one pair.
Wings and flight. Most insects can fly at some point in the life cycle; flight is thought to be one of the characteristics of the common ancestor of all insects. On the other hand, many insects have long, flightless larval or nymphal stages, so you'll find plenty of insects that lack wings. Other arthropod don't fly.
Gas exchange organs: Tracheae. The tracheal breathing system is one of the key adaptations enabling insects to live on land. Some insects have aquatic larval forms, which have gills.

Later, we'll do a lab on identifying insect orders.

Grasshopper dissection

We have some preserved lubber grasshoppers (Romalea) for you to dissect. These big insects provide a good introduction to the overall arthropod body plan. You've already learned about the grasshopper's structures involved in osmoregulation and excretion, gas exchange, and digestion. You should be able to see some of these features in the specimens we have in lab.

Try to find the following features in the grasshopper:

Main body tagmata: head, thorax, abdomen.
Digestive system: crop, gizzard, gastric ceca.
Osmoregulation & excretion: Malphigian tubules.
Gas exchange: spiracles, tracheae.
Sensory systems: eyes, antennae, tympanum (organ for hearing, located on the side of the body).
Circulatory system: hearts.
Nervous system: ventral nerve cord & ganglia