Opisthosoma Size?

[CakeLore]

Is there a general rule of thumb for the proper size of the opisthosoma of a healthy T? Maybe with respect to the prosoma?

 

[Jterry]

Had to look up these terms to know exactly what you were talking about, ha. The opisthosoma would be the "abdomen" and the prosoma the cephalothorax, amirite? I think that a general rule of thumb would be... maybe for the opisthosoma to be around 1.5x the size of the prosoma? I'm just basically guessing here, going off of my knowledge of what a "skinny", "healthy", and "fat" tarantula would look like. Most healthy-looking T's will have an abdomen that appears to be around 1.5x the size of their prosoma, although they're often fatter prior to a molt and thinner right after one, IME. Hope this was helpful, and anyone please correct me if I'm wrong

 

[Formerphobe]

Good for you, learning/knowing the proper terms!

You will find varying opinions on this subject. IMO, an adult tarantula in mid-instar with in ideal body condition will have an opisthosoma about the same size as the prosoma or cephalothorax. Exceptions to this are: fast growing slings (larger abdomens), and either freshly molted (smaller) or due to molt (larger) more mature tarantulas, and gravid females. Huge behinds = increased risk of injury. Generally the heavier bodied terrestrials tend toward larger abdomens than arboreals.

If you ever notice recently WC adults or photos/videos of wild tarantulas in their native environs, rarely to never do they have the huge, huge rear ends of their captive kept cousins.

Fat does not equal healthy.

 

[Stan Schultz]

Is there a general rule of thumb for the proper size of the opisthosoma of a healthy T? Maybe with respect to the prosoma?

Although a lot of us have personal opinions, some of them quite bizarre, nobody knows for sure. In fact, we aren't even sure at what point a tarantula may be "obese."

The problem is the bizarre lifestyle and physiology of tarantulas. They're ambush predators that work in a very restricted territory... a circle roughly two DLS in radius, centered on their burrows. Hence, they have to be able to survive a very irregular and unpredictable food supply. They do this partly by means of a very VERY low metabolic rate. Another characteristic is their ability to eat and internally store a large amount of food material when it's available. (I'm not even sure that their stored food is fat!) And the principle reason we suffer from obesity is the stress it places on our heart. Tarantula hearts are radically different from ours, so we have no way of judging the impact of obesity on them.

The only sure things that we can say are that a tarantula that is so obese that it has trouble lifting its opisthosoma off the substrate can't be all that comfortable. And, obesity certainly isn't necessary to a tarantula's existence judging from the fact that few wild tarantulas ever are obese. (Click or right-click the thumbnail to see a larger image.)


A freshly caught Aphonopelma moderatum (Rio Grande gold tarantula) from the Laredo/Eagle Pass area of Texas. Photo by the author.)


"The magnitude of our ignorance [about tarantulas] is staggering."
- S. A. Schultz, TKG3



---------- Post added 09-12-2013 at 12:03 PM ----------

... the prosoma the cephalothorax, amirite? ...

I am SSSOOOooo glad you asked because even big time, professional arachnologists don't get this right!

"If you're going to be a smartass, first you have to be smart. Otherwise you're just an ***."
(No, that's not a criticism aimed at you or anyone else in particular. It's just an Internet joke used here for illustrative purposes.)

A cephalothorax is defined as a fused head (cephalo-) and thorax. Which means that you've got to have a head first, right? Well, there is no anatomical, embryological, or paleontological (fossil) evidence to even suggest that spiders or tarantulas, in fact most chelicerates, ever had a head. So, they can't have anything like a cephalothorax. And even Wikipedia (Heaven forefend!) got it wrong!

Even the experts sometimes fail to appreciate how radically different these creatures are!

A few arthropods do have a cephalothorax. Most crustaceans for instance. But, just because a lobster has a cephalothorax, doesn't mean you get to automagically blame it on anything else.

One of the major difficulties in both the science of arachnology and the hobby of arachnoculture is that terms for various body parts and many other qualities and characteristics of arachnids in general, spiders in particular, and tarantulas most specifically are automagically borrowed from some other arthropod group (usually insects because they're the more notorious and more studied creatures) with little or no regard (and often abject ignorance) for their origin or homology. (Contrast the biological meaning of the term analogy.)

"All right! So you're a nit picker!" you cry! YES, most emphatically. But, largely because of errors such a this, abject ignorance (see above), or even a "I don't give a **** attitude, it was necessary for me to spend a lot of time typing up the ever growing list of myths and screw-ups found here. And sooner or later, some of this misinformation is likely to cause you to make a serious error in the care of your valued and valuable little 8-legged buddies. The big problem is that we don't know which misinformation it will be. So we have to attack them all, equally.

And, I thought I was going to have time to enjoy my retirement. Sigh.


(Click or right-click the thumbnail to see a larger image.)