Small World Discoveries
by Tony Enticknap - tickspics
Focusing on insects, arachnids, fungus and other small nature subjects from East Dorset and the New Forest ...
MYRIAPODA | Post-embryonic development
It's not always apparent whether the centipede or millipede you're observing is immature and still in the process of developing, or whether it's actually an adult of one of the smaller species - sometimes it is, but very often it's not. I tried to
put together some useful pointers in respect of this matter when I wrote the Cryptozoa - 'soil organisms' article regarding 'juvenile v adult' identification, but certain species and genera can be tricky, so even though I have a reasonable understanding I'm pretty sure that there are still going to be occasions when I'll need to seek help. And, of course, the other point that must always be remembered, is that there are always going to be problems and limitations in trying to determine certain species from photos.
To improve my knowledge of the way these creatures develop I've summarised the process from various bits of information that I've managed to find. I originally had this document saved as notes, but thought it would serve better purpose here on the website. It's a bit raw and could do with some more photos that may be added in time, but hopefully it will prove useful.
Chilopoda - centipedes
Centipedes go through one of two different types of post-embryonic development processes after hatching, which are known as epimorphosis and hemianamorphosis.
In the British fauna, geophilomorphs and scolopendromorphs (Cryptops) are epimorphic centipedes, characterised by having the full quota of body segments and leg pairs when they hatch, but still need to go through a further series of moults before they reach maturity. Visually they may look like adults, but they are still in the process of developing as various features and structures are not fully pronounced.
The lithobiomorphs (Lithobius) species on the other hand are anamorphic, where the earliest stages after hatching have fewer body segments and leg pairs than the adults. They develop through hemianamorphosis where additional legs are formed from limb buds during a series of moults until the requisite number of fifteen pairs is achieved, but with subsequent moults taking place while the centipede grows and sexually develops.
The two phases of growth are generally known as the 'larval' stage characterised by incomplete adult segments and leg pairs, and the 'post larval' stage when an adult-like appearance is achieved, but where the individual is still developing, and where the common term of juvenile is usually used, although later stages could also be referred to as subadults. Scientifically the first post-embryonic stage is now regarded as anamorphic and the second stage as epimorphic relating to the fact that whilst individuals may now look like miniature versions of the adult, they're still growing and sexually immature without fully developed genitalia or forcipules.
Still referring to the lithobiomorphs, the first development phase noted above typically consists of five defined stages starting with the hatchling, anamorph A0, which has seven leg pairs, followed by A1 to A4 for each subsequent moult, when additional leg pairs are added in a predictable pattern such that the actual growth stage can be determined. Similarly, with the second epimorphosis phase, which is again recognised as having five distinct stages with names like AG - agenitalis, IM - immaturus etc., up to M - maturus for the final fully mature stage.
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Diplopoda - millipedes
Most baby millipedes possess just six body segments and three pairs of legs when they're born, with subsequent segments and leg pairs being added between the penultimate ring and the telson during a series of moults as they develop through the post-embryonic growth stage of their lifecycle - a process known as anamorphosis.
In millipedes, the term stadium or stadia is used to describe the different growth stages or interval of time between moults, rather than instar which is more appropriately used when referring to the physical form of the species at any given stage.
Three different types of anamorphosis are recognised in millipedes; hemianamorphosis where segments and leg pairs are added until the species-specific number is reached, known as the final stadium, but with further moults taking place without new segments being added; teloanamorphosis, which is similar, except that moulting ceases after the final stadium is reached; and lastly, euanamorphosis which occurs in certain species that continue to grow throughout their entire lives, moulting and adding further segments and legs even after reaching sexual maturity.
Chordeumatids and polydesmids develop through teloanamorphosis and have a given number of body segments or rings, 30 in the former, 19 or 20 in the latter. The increment of new diplosegments at any given moult is always the same, one to four in Chordeumatida over eight or nine stadia, and one to three in Polydesmida over seven or eight stadia. These millipedes are sexually mature adults at the last stadium.
The anatomical term of body segment or body ring are somewhat interchangeable, but segment is best used when referring to flat-backed millipedes and ring for the cylindrical species. However, ring is also used as a general term when counting the body segments. In respect of polydesmids, adults Polydesmus species would have 20 rings starting with the collum directly behind the head + 3 haplosegments (effectively having three pairs of legs) + 14 diplosegments (each with a pair of legs, except in males where the anterior pair on ring seven are formed into gonopods) + an apodous (legless) segment at the tail-end, and finally the telson which counts as the final segment. In Brachydesmus superus there would be 13 diplosegments, therefore 19 rings in total. Individuals with 18 rings or less are not yet fully developed.
In Julida, increments of new rings vary from one to eight and the increment is not always the same for a given stadium; thus the stadium cannot be determined simply by counting the number of rings. A given number of rings may be the result of a smaller number of larger increments or a larger number of smaller increments. Adults do not have a fixed number of species-specific body rings as they develop via euanamorphosis which means that older individuals are significantly longer and possess more legs than younger adults of the same species. The number of leg pairs are therefore variable, but are always an odd number. The number of stadia vary from six to fourteen or fifteen, with maturity achieved in any stadium from one to five for any given species, but where that stadium may or may not be the last.
If you wanted to count the number of rings, you'd start with the collum, which is legless, but is still regarded as the first podous ring when counting, then 3 haplosegments (sharing three pairs of legs), followed by a variably large number of fused double-segmented diplosegments each bearing two pairs of legs, and then another undefined number of legless apodous rings before the telson. It's easy to lose track, but in many cases the ozadines (as noted below) help the process as you can count up to where they start, then the number of ozadines which are on leg-bearing segments, followed by the non-bearing segments and telson. The formula is often simplified as PR + AR + T.
Although the stadium can't be reliably confirmed from the number of body rings, it's interesting to note that in these species the ocular field grows regularly at each moult with new ocelli added in a definite sequence such that, with close inspection, the stadium could be determined by carefully examining their number and arrangement. The first stadium is blind, the second has one single ocellus, then a row of two in front of the single ocellus in the third stadium, and a further row of three ocelli added in the fourth.
A further development feature of the Julida is that the internal stink glands (ozadines) extend from body ring five or six until and including the last leg-bearing ring. They are particularly visible in lightly pigmented species and in young individuals of darker species where the glands shine through the cuticle appearing as dark, often reddish spots. In darker or older individuals they may not be visible.
In certain species, notably Blaniulus guttulatus, Ommatoiulus sabulosus and Tachypodoiulus niger, sexual maturity of males occurs before the final stadium (growth stage) known as periodomorphisis where the individual has an infertile 'intercalary' stage with regressed gonopods to survive harsh environmental conditions and then, during the spring moult, returns to a sexually mature form. They still continue to develop through his stage with new segments being added thereby retaining the euanamorphic growth pattern.
One final point that had me thinking about the tiny immature individual in the following photo that was featured in the Cryptozoa - 'soil organisms' article regarding 'juvenile v adult' identification, is in respect of the last few body segments being legless, when new growth segments are added to the rear of the animal between the penultimate ring at the telson. I could understand that the lack of legs was a good visual indicator for confirming that this was an immature millipede, but didn't realise that formation of the legs was a secondary process. This starts at the hatchling stage as the initial six segments comprise four podous rings with three pairs of legs between them, followed by two apodous rings, which effectively sprout legs during the following moult. Subsequent moults add various numbers of further apodous rings, from which new legs will appear, so I gather that this particular individual is at that stage where it appears to have added five new segments, but now needs to go through a further moult before functional legs become apparent.
However, to add one last layer of confusion, adult julids typically have a couple, or possibly as many as five, apodous rings before the telson, so a judgement still needs to be made whether the lack of legs at the rear end is indicative of an individual that is still developing or an adult that is still growing and adding legs.
An expert on the BMIG FB group thought that this immature julid was very possibly Tachypodoiulus niger as the juveniles of the species typically have this pale brown colouration. I followed this up as I have a couple of well-detailed photos and was able to confirm that there x30 body rings (x22 podous + x7 apodous + the telson). I was also able to accurately check the legs at x37 pairs. The associated details for the species would put this individual midway between stadia IV and V, confirmed as "lighter brown with heavier pigment along the line of the ozodenes and in a median dorsal dark line". They darken up, becoming black at stadium VII and are sexually mature at VIII.
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Page added, Sept.26 (v.1)