This diary may seem very very distant from the mission of Daily Kos. My plan is that this is going to be the first in a series of diaries that summarize the diversity of arthropods. WTF! - many of you may be thinking right now, assuming anyone is actually reading this in the first place. Patience gentle reader, there is method to my madness.
One of the most important things we can do in the face of global change is document the changes we see in the natural world. One of the keys to accurate documentation is knowing what you are looking at. Arthropods are one of the major components of biodiversity that the average person will encounter in their day to day lives. I'm going to present a brief survey of the diversity of arthropods that will hopefully give those interested a place to start when identifying cool critters you've found. This diary will focus on the major groups of arthropods as well as an overview of non-insect terrestrial arthropods. The next diary will be an overview of the major insect groups (orders) followed by future diaries on individual insect orders.
I'm going to try and design these diaries as companions to the excellent Bugguide website. This site is an astonishing resource with a myriad of user contributed photos and information on localities for an enormous number of North American insects. What the site lacks is an easy way to narrow your search. If, for example, you have a beetle but have no idea what family it is in, you could be spending hours and hours looking through photos until you find something that matches. Or worse yet, you find something completely unfamiliar and you have no idea where to start at all.
The Backyard Science group here on Daily Kos has made use of the Bugguide site and my initial motivation for writing this is to help my fellow BYSers use the site more effectively. But I hope these diaries can be useful for anyone interested in the diversity of life around them.
This diary will follow the classification scheme on Bugguide so that hopefully it can be of some use in getting started. One important note: the actual rank (class, order, family, etc.) of a taxon (named group such as Insecta) is partly arbitrary. Orders are always subsets of Classes but there is no objective standard by which the Class Insecta and the Class Mammalia are biologically equivalent. Especially at the higher levels different classification schemes have been proposed many times and in some cases something considered an Order in one source might be a sub-class in another. Don't worry about it too much. For those of you with some expertise I think this introductory diary will large be superfluous but I will try and write some more specialized ones down the road.
Tiger Beetle - a 'typical' Arthropod
A few general caveats before we begin
1. In the beginning I'm going to focus on adult animals. Once we have that framework in place we can talk about juvenile forms.
2. There are always going to be some specialized forms that don't fit the general pattern. Many parasitic forms or some highly specialized plant feeders like scale insects are going to be quite misleading.
3. I'm also leaving out some groups that are extremely unlikely to be encountered - for example there are three orders of Arachnids that are both tiny and rare.
Part One: Introduction to Arthropods. The Phylum Arthropoda (the Arthropods), is the largest group of animals on earth in terms of number of named species. There are approximately 20 known species of Arthropods for every vertebrate species. And there are huge numbers of unnamed arthropods. This has major consequences for studying arthropod diversity both at the professional or the amateur level. There are about 10,000 species of birds and virtually all of them are covered in field guides. Experts on just about every group of birds abound. In contrast a single family of insects may have many more than 10,000 species and perhaps only a handful of experts who have comprehensive knowledge of the group. Field guides to insects typically cover the relatively small number of large and conspicuous insects (e.g. butterflies and dragonflies). Hence bugguide and hence this series.
Arthropods are characterized by having an external skeleton (exoskeleton) and legs with joints. Arthropods also have highly segmented bodies. A lot of arthropod evolution has occurred through a process called tagmosis where segments fuse to form larger more complex structures and the appendages associated with the segments take on specialized roles such as antennae or mouth parts. It was thought that the ancestor of vertebrates was a segmented, worm-like animal and that tagmosis has resulted in the diversity of body forms we see today.
Arthropods have been traditionally organized into four more main groups (Subphyla). Recent research indicates that this classification is a bit artificial but it is well known and convenient and only slightly incorrect. We are going to look at the following criteria when grouping the arthropods into subphyla.
1) Antennae - does the animal have 0, 1, or 2 pairs of antennae? Antennae are sensory structures (modified appendages) found on the 'head'.
2) Number of body parts/regions. As mentioned above many segments are often organized into a larger body region such as a head. Each group (other than the uncooperative Crustaceans) has a characteristic number of body regions.
3) Number of walking legs
4) Mouth parts - I'll mention this but this can be hard to determine without a microscope so it may not be of much use.
Part Two: The Four Subphyla
A) Chelicerata - Have no antennae and the body is organized into two major parts (these are further fused into a single body region in some groups such as mites and daddy longlegs). The front part has the mouth, eyes, and walking appendages. The back section has no appendages and contains most of the internal organs. There are four pairs of walking legs and two pairs of appendages associated with the mouth region. The chelicerae are eating structures and the pedipalps are more leg-like and used for both sensory purposes and manipulating food (the claws of scorpions are pedipalps). The Chelicerata contains two small groups of marine organisms (the horseshoe 'crabs' and the sea 'spiders') and one very large group of terrestrial (and some freshwater) organisms. The latter are the Arachnids which contains the spiders, mites and ticks, daddy longlegs, scorpions, and a number of other groups less well known to the average person.
Tarantula - showing the two body parts, four pairs of walking legs, pedipalps (extending straight forward) and chelicerae (in the center).
B) Myriapoda - Have a head with one pair of antennae and mouth parts similar to those of insects and crustaceans. The rest of body is a series of repeated segments with walking legs, ranging from a dozen or so segments to over 200. The two well known groups of Myriapods are Centipedes (one pair of legs per segment) and Millipedes (two pairs of legs per segment). All living species are terrestrial. This is the smallest of the four groups in terms of number of species and the least diverse in form and ecology.
Millipede. Note the two pairs of legs for each segment.
C) Crustacea - The insects of the sea. Primarily marine and freshwater group with a limited number of land living forms. More variable in body structure than any of the other groups. They have two pairs of antennae although these may be very small in some crustaceans such as crabs. In addition to the familiar shrimp, lobster, and crabs this group contains a large number of very small forms that are planktonic, live in the sediment, or in other less than obvious locations. By far the most widespread and familiar land-living crustaceans are the terrestrial isopods known as roly-polies, pillbugs, sowbugs, or woodlice.
Hermit Crab - note the two pairs of antennae
D) Hexapoda- This group is by far the largest in terms of number of species and contains the Insects and a few other rather obscure groups. All of them have a body divided into three major regions: a head (sensory structures), thorax (walking legs and wings), and abdomen. Theyhave three pairs of walking legs and a single pair of antennae (the antennae have been lost in a few rare cases). Many have wings, a trait that is absent in all other arthropods.
A midge (fly) and a camel cricket showing the three body parts and three pairs of legs. The mites on the midge are bonus arachnids.
Part Three: Where are they found?
This can help narrow your search.
Chelicerata - The two small groups, Horseshoe Crabs and Sea Spiders are exclusively marine. The large group Arachnida is mostly terrestrial but does have some species in fresh water.
Myriapoda - Living forms are exclusively terrestrial to my knowledge
Crustacea - Highly diverse in marine environments. Diverse in freshwater as well although there are many fewer large species compared to marine environments. Away from the vicinity of the ocean there are few terrestrial crustaceans. The one widespread group are the terrestrial isopods (there are marine and freshewater isopods as well).
Hexapoda - Highly diverse in terrestrial environments. Also common and diverse in freshwater. Absent from saltwater except for a few intertidal species.
Identification to Subphylum
I'm going to include two different sets of questions. The first will be used in strictly terrestrial situations in which the only crustaceans likely to be seen are isopods. The second will be a more general key. Note that this is for adults only.
1. Specimen is a land-living form (ignores land crabs).
Has wings = insect (Hexapoda)
Has three body regions,one pair of antennae and three pairs of legs = Hexapoda
Has a head followed by a longish many segmented body and seven pairs of walking legs = Crustacea
Has a head followed by a long many segmented body with at least 15 pairs of walking legs = Myriapoda
Has one or two body regions, four pairs of walking legs and no antennae = Chelicerata (Arachnida).
2. General Identification
Has wings = insect (Hexapoda)
Has three body regions,one pair of antennae and three pairs of legs = Hexapoda
Has two pairs of antennae (can be hard to see in many cases), body form highly variable (if two major regions, similar to Chelicerata then frequently small paired appendages on the abdomen) = Crustacea
Has a head followed by a long many segmented body with at least 15 pairs of walking legs = Myriapoda
Has one or two body regions, four pairs of walking legs, no appendages on the back body region and no antennae = Chelicerata.
Classes of the Chelicerata
The Chelicerata typically have four pairs of walking legs, a pair appendages called pedipalps which are usually associated with feeding and are located between the walking legs and the mouth. The chelicerae are the sixth pair of appendages and form the jaws. All of these appendages are attached to the front body region, the cephalothorax. The hind region is the abdomen.
Merostomata - Horseshoe crabs. Large animals with limbs and mouthparts underneath, covered by the extensive upper exoskeleton. Has a hard, pointed tail-like structure. The four pairs or walking legs and the pedipalps are similar in size and shape (male pedipalps have hooks for grasping femailes). Very few species, only in salt water (come onto beaches to mate).
Pycnogonida - Sea Spiders (not in Bugguide). Very odd spiderlike marine animals. Have extremely long legs and minute bodies (abdomen is almost completely absent). Small and live in algae and colonial marine invertebrates. Probably more common than realized but very inconspicuous.
Arachnida - Spiders, mites, and relatives. Very large primarily terrestrial group. Given the distinctive nature of the other two Chelicerate groups there is not likely to be any confusion.
Orders of the Arachnida
Things to look for are fusion of the body regions, segmentation and shape of the abdomen, size and form of the pedipalps.
The first four Orders are the most commonly encountered.
Order Araneae (Spiders) Body in two distinct regions, abdomen is usually not segmented, abdomen has fingerlike spinnerets at the end, pedipalps are small.
Spider - showing body regions, walking legs and pedipalps
Subclass Acari (Mites and Ticks) Two body regions are broadly joined and indistinct from one another. Abdomen is usually not segmented. Pedipalps are small. Legs are fairly short relative to overall size. Body size is small to microscopic.
Order Opiliones (Daddy longlegs)Two body regions are broadly joined and indistinct from one another. Abdomen is obviously segmented. Legs are very long relative to size of body, pedipalps are small.
Tropical Opiliones - showing fusion of body regions and segmentation on abdomen as well as wild colors.
Order Scorpiones (Scorpions)Two body regions are distinct. Abdomen is long and slender with distinct segments and a stinger at the end. Pedipalps are larger and crab claw-like.
The other four orders are less commonly encountered but include some very striking and distinctive animals.
Order Pseudoscorpiones (Pseudoscorpions). Body is scorpion-like but with a blunt, round-ended abdomen. Pedipalps are slender and crab claw-like. Usually very small. These are actually quite common animals but are very small and often overlooked.
Order Uropygi (Whipscorpions) - Body is scorpion-like. Abdomen is sausage-shaped with a long, whiplike appendage on the end rather than a stinger. Pedipalps are crab claw like (short and stout). First pair of walking legs is long and slender. Large and impressive (i.e. scary-looking) animals.
Order Amblypygi- Tail-less Whipscorpions (sometimes called Whip-spiders which is a better name as they do not look like scorpions) - Body is spider-like but highly flattened and with a segmented abdomen. The pedipalps are large and have very large spines on the last two segments for capturing prey. Front pair of walking legs is long and slender.
Order Solifugae (Wind/Sun Scorpions/Spiders). - Body is spider-like with a segmented abdomen. Pedipalps are large (larger than walking legs) but not claw like. Mouthparts (jaws) are very large and powerful. Very rapidly moving and ground living, most in arid areas
Myriapoda Groups
There are four classes of Myriapoda. Two of them, the Symphyta and the Pauropoda are very small animals found in the soil or among debris on the ground. You are unlikely to come across them unless you look for them. They generally resemble centipedes and millipedes and have more walking legs than any non-Myriapod.
Otherwise.
Class Diplopoda - Millipedes. Segments are fused such that each segment has two pairs of walking legs. Legs short relative to animal's body size. Usually slow moving animals. Body is often dome shaped in cross section.
Class Chilopoda - Centipedes. One pair of walking legs per segment. Legs are often relative long. Swift moving animals. Body is usually flattened in cross section.
Order of Centipedes
Order Scutigeromorpha (House Centipedes) - Very long legs (15 pairs) that rise up noticeably higher than the animal's back and then arch down to the ground. Very rapid locomotion. Usually light colored. Commonly found in houses.
Order Geophilomorpha (Soil Centipedes) - Very slender centipedes (almost wormlike) with 29 or more pairs of legs that are found in the soil. Very thin for their length.
Orders Lithobiomorpha (Stone Centipedes) and Scolopendromorpha (Tropical Centipedes) - These two orders both have the typical centipede appearance. The stone centipedes have 15 pairs of legs and are usually a red/orange/brown color. They are probably the most commonly seen centipedes, commonly found under objects.
The Tropical Centipedes are similar in general appearance but have 21-23 pairs of legs, are often very large and variable in color. They can have very painful bites and pinches (from the hind pair of legs).
There are ten orders of Millipedes and the differences between them are not so obvious (and I am not familiar with them). I'll save them (and the Crustaceans) for another diary.