I am designing a 'fairy' that is a relative of primates (essentially a sapient flying monkey). It has a skeleton similar to that of a spider monkey, but with a larger ribcage and shorter limbs. It will have a third pair of limbs in the form of wings just behind the forelimbs.The wings will neither be feathered or furred, but be similar to bat wings. They are comprised of membrane tissue webbed between "fingers", and the first and last two fingers will be fused so that the fairy's wings each have three sections (thumb, first two fingers, and last two fingers). ** Where should I place the secondary scapula for these wings? And how should they attach to the clavicle (if at all)?**
Google for "Anatomy of angel wings" in google images. You will find some interesting anatomical diagrams where people analyze musculature and skeletal systems for "wings and arms"-type creatures. Wings are anatomically a variant of arms and hands.
Based on my research, there are effectively four variants you will find:
- it's magical, so just trust me, it works. This is a viable solution.
- wings/arms stack on top of each other as if the creature has two shoulders. This is anatomically insanely difficult to imagine, though aesthetically looks cool.
- wings/arms stack front-to-back. - viable, though would make for a massive musculature more appropriate for creatures rather than humanoids or "slight" creatures like monkey-fairies.
- mid-back wings - scapula is essentially a fusion of some ribs behind the lungs. This is probably the most "acceptable" placement and easiest to explain to science-minded readers.
The wings usually dont have much scapula. Take a look at this chicken skeleton for example:
It just gets in the way of flying to have much scapula. But as I have remarked in other questions about 4 limbs plus 2 wing creatures it is easier to use kinematic chains.
Having wings below the arm scapula means that they are supported by the floating ribs (if you want to leave room for the arm scapula to rotate and translate downwards) and the musculature of the lumbar region with no real connection to the clavicule. To generate enough lift to push the spidermonkey in the air the muscles at the stomach and back have to generate a force larger than the monkey's weight, without a skeleton to support that properly you would either have to painfully smash the intestines, kidney and lower Aorta each time you flap the wings, connect the muscles to the hips and sternum causing a bending motion with each flap or add skeleton structure that severely hampers the bending capability of the monkey.
This leaves putting the wings on top/between the arm scapula. You already mention the chest would be larger, which I hope means larger sternum like the overlarge sternum seen in the chicken picture. Here the kinematic chain comes into effect. Muscles can stretch over multiple joints which allows forces to be transferred from one joint to another. For example if the knee is bent but you counter the force with the quadriceps then your hips are pulled forwards. This is what you can use with sit-ups when someone sits on your feet. A similar kinematic chain can be used for the wings. It does mean the wings and arms will not move independently when flying and have limited independent movement from the arms when not flying.