Arthropods, the animals with exoskeletons, paired jointed limbs of varying number and a segmented body. There are by far more Arthropods in the world than any other type of animal and this is clearly reflected in the Garden.

The classes of arthropods currently recorded in the Garden are Arachnida (Spiders, Harvestmen and mites), Branchiopoda (Water Fleas), Chilopoda (Centipedes), Collembola (Springtails), Entognatha (Bristletails), Hexanauplia (Copepoda), Insecta (Insects), Malacostraca (Woodlice, Water Slaters), Ostracoda (Ostracods). There are 100’s of arthropods recorded in the Garden so we have broken them down into these Classes below…

What are arthropods?

The Animal Kingdom is divided into many phyla, scientists are still debating how many. The Arthropoda is just one such phyla. You may also come across the term ‘Invertebrates’, most often associated with Arthropods, in particular the class Insects. In fact the term Invertebrates relates to all animal life without a backbone. Not just the various classes of Arthropods (as listed above) but other phyla as well, such as Molluscs (slugs and snails) and Annelids (worms). Indeed the only phyla not considered an Invertebrate are the Chordates, to which ourselves and all mammals belong, alongside birds, reptiles and amphibians.

The phyla Arthropoda are therefore “Invertebrates” and they’ve been on this planet most likely for the last 550 – 500 million years. To put that into perspective, this was so long ago that the Wildlife Garden, along with most of England and Wales were part of Gondwanaland located near the Antarctic circle. To be clear, we mean by “on this planet” is that animals like todays arthropods (their ancestors), sharing several similar characteristics, began to appear in the fossil record around this period in Earths history and from which todays arthropods likely evolved.

In all that time it is hardly surprising they have evolved in many different ways, responding to many different and changing environmental pressures, and through natural selection have diversified into the wide array of different forms we see today. But what is it that todays arthropods have in common with each other and such ancient creatures and what sets them apart from other phyla such as Molluscs (Slugs and Snails) and Annelids (Worms)?

Arthropods, including those Classes shown above and recorded in the Garden all share the following characteristics;

• An exoskeleton
• A segmented body
• Paired and jointed appendages
• Bilaterally symmetrical

Google something like ‘Oldest known arthropod’ and provided the result isn’t someone boasting about Alan their 24yr old Tarantula. Then something like a fossilised Trilobite will pop up. This ancient fossil, from around half a billion years ago was considered to be an early ancestor to today’s arthropods. Having all four of the characteristics listed above.

There are other similarities between arthropods, but these are the most obvious, and when identifying wildlife in the garden it is often interesting and useful to understand these similarities.

1)The Exoskeleton: Unlike mammals or birds (non-invertebrates) arthropods don’t have a skeleton on the inside acting like a frame upon which muscles and ligaments can be attached. Arthropods all have their skeletons on the outside, more like armour than a frame. The individuals below all belong to different classes of the Arthropoda (Arachnid, Insect and Malacostraca respectively) but all clearly have an Exoskeleton (a hard outer casing if you prefer).

Other invertebrates don’t have the same kind of armour as an arthropod. Annelids for example don’t have a similar structure at all, being worms and soft bodied.

Whereas molluscs can have a partial hard covering, such as the shell on a snail. However there is a difference. The shell of a mollusc continually grows, allowing the animal inside to grow with it. The shell never needs to be shed.

Back to arthropods and taking this odd looking tiny insect below recorded in the Garden as an example.

This is the insect in its first stage after hatching from its egg. Each stage is termed an Instar, so this is it’s 1^st Instar. This nymph (small version of the eventual adult) will need to shed this dark green/black exoskeleton in order to grow.

In its 2^nd Instar below, we can see that it has shed its original exoskeleton and is now significantly larger and looks a little different.

Note 1: It can be confusing to understand how shedding an exoskeleton will allow the insect inside to grow, surely the replacement skeleton will be the same size as the animal shedding it. Fortunately this is not the case. Once the insect is free of the old exoskeleton, it remains soft for a period of time and has the ability to temporarily expand its body size before the exoskeleton on it’s outside can set hard. Once the skeleton is set the body shrinks to its actual size and there is then room within the skeleton for the animal to permanently grow.

Note 2: That not all arthropods go through the Instar process in the Nymph to Adult transition, which is a process known as incomplete metamorphosis. Some go through what is known as complete metamorphosis. These for example include some insects which hatch not into a mini version of the adult (a nymph) but into a grub or caterpillar, something that in no way resembles the adult and is instead called a larva. Species that go through complete metamorphosis (Bees with their grubs, Butterflies with their caterpillars, True Flies with their maggots etc) have their Instar stages at the larva to pupa phase. So, for example, caterpillars shed their skin periodically until they are large enough to pupate and which stage they metamorphosis into a butterfly which will never grow any larger.

The above insect, a very familiar garden visitor, will go through 5 instars in total. With each moult it will look more and more like the adult it will hopefully survive to become. Following the final Instar it sheds its exoskeleton a final time and emerges as a Common Shield Bug.

2) Segmented body: Our friend the shield bug above is of course an insect, one class of many that make up the Arthropoda. All insects have three body parts (segments), the Head, the Thorax and the Abdomen, although in the case of the shield bug these are not always that obvious. A clearer example of this is seen in the Common Wasp, as perhaps are the functions of the three segments. The head supports the mouth parts, the antenna and the eyes. The Thorax the three pairs of legs and the wings (two pairs in the case of wasps). And finally the abdomen which in this case contains that famous stinger.

However although the rule for arthropods is they have segmented bodies, this isn’t to say they all have to have three segments. For example spiders, which as we know are not insects but are nether the less still arthropods. They have only two body segments (they also never have wings), and these are called the cephalothorax and abdomen. The cephalothorax being a merged head and thorax and, in this case, supporting 4 pairs of legs instead of three found in insects. The abdomen contains the animals reproductive parts and spinnerets for making webs.

So whilst all arthropods have segmented bodies, not all have the same number of segments, think of centipedes as an extreme example.

3) Paired and jointed appendages: It’s very easy to see this in the above examples. Looking at the legs of the insects and arachnids above you can see that they come in pairs and are clearly jointed. But to clarify, jointed appendages in the sense we mean here also includes the antenna and mouthparts such as the mandibles etc. Antenna being the most obvious example, especially in arthropods such as the Ants.

4)Bilaterally symmetrical: Simply put there is only one direction you can chop an arthropod in two (bi) and have a symmetrical (identical) pair of halves. As shown by the red line in this image of our most welcome visitor the White Legged Damselfly. Chop this fella along any other plane and the two parts won’t look the same. This is not the same for the non-arthropod star fish, which can be cut in more than one direction and result in identical halves.

In summary: Many other types of animals have one or more of the above characteristics but none exhibit all of them except the arthropods. Worms are bilaterally symmetrical and mostly segmented, but they don’t have an exoskeleton or jointed limbs, and therefore are put into a different phlya (the annelids with whom they share a different set of characteristics). Mammals are bilaterally symmetrical, and they do have jointed limbs, but not always paired, elephants only have one trunk for example. They certainly don’t have an exoskeleton, although the Armadillo tries it’s best. They also happen to have a backbone which as explained above means they are not even invertebrates.  

Clearly the combination of these combined characteristics, shared by all arthropods has been a very successful design.

What are the arthropods doing in the garden?

At the phyla level and in particular with such a diverse phyla as the Arthropoda, this is an impossible question to answer, unless an acceptable answer is ‘nearly everything that should be happening’.

Arthropods range from herbivores which attack our shrubs, to obligate carnivores, which prey on other animals. From scavengers to hunters, graziers to parasites. These guys cover all the ecological requirements of the garden, whether that’s in the soil breaking down organic matter into smaller and smaller fragments, helping fungi and bacteria to complete the process of nutrient recycling. To feeding on or off of pests (not a phrase we pay any attention to ourselves), to forming the basis of a food chain that extends up to the apex species, ourselves included. Some like the ants are even livestock farmers!!! In this case tending aphids for their sweet honeydew.

Many are just visitors, many are permanent residents, whilst others may stick around as long as we encourage them to do so. They inhabit the tops of the trees, to below the soil surface, the larva often surviving in totally different situations than the adults.

They live on and in plants, some preferring live plants whilst others favour dead plant material. They live on and in other animals also, the host animal may be dead or alive at the time depending on the arthropod. Some benefit the host, some do not, others don’t seem to make a difference.  Many live on and/or below the surface of the water. They make nests, form galls, roll leaves or simply drop eggs at random. Some like woodlice care for their young, others not so much.

Importantly some are welcomed by humans. Bees and ladybirds for example as the latter pollinates plants to our advantage (and of course the plants) or in the case of ladybirds attack those arthropods that then try to eat the plants. Note however that whilst bees get all the credit for pollination, lots of arthropods, in particular insects, help pollinate plants, flies for example.

Other less welcome species (pests) are often the focus of our physical and chemical warfare, which like all warfare does little net good for anyone or anything in the end.

Quite simply without Arthropods the garden would not and could not function. There is no point in us disliking them. They don’t care if humans exist in harmony with them or not, but they probably should….

and so should we!