Basic groups 7: Animals IV

Now we have reached what I like to call the ‘higher invertebrates’. We will look at the arthropods – the most diverse animal phyla ever, and therefore arguably the most successful animals. Since I feel my eyes are not as recovered as I thought, I will save the echinoderms, which includes starfish and sea urchins, relatively closely related to vertebrates, for tomorrow. Sorry for that!

Arthropoda is closely related to the nematode worms (see Part 5), united by the shared process of moulting – shedding their outer protective layer as the body grows. Arthropods are well characterised by their jointed exoskeleton composed mainly of chitin (a complex sugar) and segmented body. Moreover, they are bilaterally symmetrical, triploblastic, coelomate animals (see Part 4).

Previously, arthropods were actually thought to be close relatives of annelid worms, rather than nematodes, because they are both segmented, have similar nerve cord arrangement, and hearts located in similar positions. Moreover, the velvet worms seemed to make a perfect intermediate form between annelids and arthropods. However, genetic information suggests that the common features rather are the result of convergent evolution (which I’m sure you are familiar with), and that the true ancestry of the arthropods is shared with the nematodes.

We all know what arthropods are: insects, crustaceans (shrimps, lobsters, crabs and the lot), centipedes and millipedes, and spiders and scorpions. These subgroups are very different in their details, and there is enormous diversity within them, so we should take a closer look at them.

Crustacea includes all arthropods that have two pairs of antennae and biramous limbs (branched in two). Their heads are also fused to the thorax (where most limbs come out from), forming a structure called cephalothorax.

Familiar examples are shrimps, crayfish, lobsters, crabs, etc., and less typical forms include planktonic groups (i.e. really tiny ones floating around in the water column), such as the copepods and the cladocerans.

Daphnia, a typical planktonic cladoceran crustacean. 

Image from https://wiki.cgb.indiana.edu/display/DGC/Images+Set+2

Strictly speaking, crustacean is defined by a unique larval stage called the nauplius larva, which is… uhh… ugly…

Nauplius larva, a unique shared larval stage of the Crustacea. 

Image from http://commons.wikimedia.org/wiki/File:Fairy_shrimp_larva.jpg

Crustaceans typically have more limbs than anyone could count, many of which carry sensory structures, detecting chemicals (taste) or pressure changes (touch). They also have compound eyes, which, unlike ours, have a myriad of lenses, each producing its own image, and connected to all others in the brain to give a single picture in their head (I guess… I don’t really know how a shrimp sees, hahaha…). The two pairs of antennae are also involved in sense perception, so you can see that crustaceans have a plethora of sensory organs.

So many legs! Image from http://www.flickr.com/photos/dwysiu/434088344/

Cheliceriformes is a mouthful, and includes all eight-legged arthropods. They have six limb pairs in total, the last pair being used for feeding, either for grasping, like in scorpions, or piercing, as in spiders. Instead of antennae, they also have an extra pair sensory appendages, called pedipalps, near the mouth. Like the crustaceans, the cheliceriforms have a cephalothorax, a fused head and thorax, where the six limbs are attached; the rest of the body has relatively few appendages.

Scorpion. Image from

 http://connectnigeria.com/articles/2012/10/08/discover-nigeria-an-army-general-called-the-black-scorpion/

Cheliceriformes includes Merostomata (horseshoe crabs and king crabs, though none of them are actual crabs) and Arachnida (spiders, scorpions, plus ticks and mites… nasty creatures). The horseshoe crabs are remarkable examples of living fossils, having persisted virtually unchanged for more than 400 million years.

Horseshoe crab. Image from http://thenotsosilentspring.blogspot.co.uk/2013/07/the-10-eyed-horseshoe-crab.html

We would all know spiders and scorpions if we saw them. Ticks and mites are so tiny you will probably never (want to) see them, and if you do, just avoid them, for the good of everyone. They should not be encouraged to exist. (Just joking, not being serious!)

Spiders and merostomatans, unlike scorpions and the others, are sectorial feeders, which means that they can only take in food as a liquid. They must therefore digest the food before they suck it up. Spiders inject digestive juices into their victims, literally dissolving them from inside out. This is probably why it is handy to trap the prey in webs.

Myriapoda is the group that comprises centipedes (Chilopoda) and millipedes (Diplopoda). They are closer relatives of insects than the rest of the arthropods, having heads separate from the trunk, a single pair of antennae, and one pair of mandibles (feeding appendages). The main difference between insects and myriapods is the number of segments: whereas insects reduced their numbers to only three segments, the myriapods have too many to count.

Centipedes have one leg pair per segment, millipedes have two. This is a certain way of distinguishing between the two. However, you might want to be able to tell them apart before getting too close, because centipedes are vicious, often poisonous predators, while millipedes are harmless herbivores. Other useful characteristics are the length of the antennae – centipedes have very long ones, while the millipede antennae are stubby – and the poison fangs of the centipedes, which evolved from modified front legs.

Centipede (Chilopoda). Image from http://en.wikipedia.org/wiki/Centipede

Centipede fangs. Image from http://carnivoraforum.com/topic/9680654/3/

Millipede (Diplopoda). Image from http://topicstock.pantip.com/wahkor/topicstock/2011/11/X11358397/X11358397.html

Myriapods are restricted to moist environments, because their exoskeleton is less advanced than that of other land-based arthropods. They lack a waxy layer, which helps conserve water, so they need to be in damp areas in order to not dry out.

Insecta, as already mentioned, are distinguished by having three distinct segments: the cephalon (head), where the mouth, eyes and antennae are, the thorax, where the three leg pairs are attached, and the abdomen, where the most internal organs are concentrated, and which lacks appendages. Many insects also have wings – typically two pairs – on their thorax. Insects are the only flying arthropods, and, indeed, the only flying invertebrates.

Insecta. Image from http://www.naturephoto-cz.com/wasp-photo-4260.html

Insects are incredibly diverse, including flies, dragonflies, mosquitos, wasps, bees, bumblebees, grasshoppers, crickets, all sorts of beetles, butterflies, moths, ants… and many more that I just can’t think of right now! There’s so many of them! And they are everywhere. I wish I knew more about insects. They could probably have made up a post like these on their own!