Inspired by the 1986 version of The Fly, or the webcomic Spinnerette. It’s easy to imagine simply adding extra limbs onto a person but I was wondering what the result would really be if you could “average” the anatomy of an insect or spider with a vertebrate. The problem is that other than they’re both Bilateria, arthropods and chordates don’t have much in common even at the embryonic level. AFAIK the limbs in particular develop from totally different structures. Does anyone here know more about this?
A couple things from my memory.
Although limbs and other organ may arise from different structures, some of the same basic genetics are involved. For example, the developmental gene that switches on to tell the developing embryo where to grow eyes is essentially the same in both arthropods and chordates, even though the eyes themselves are completely different in design. So the differences in embryonic development might not be completely insurmountable.
One major difference is that the dorsal-ventral axis is reversed in vertebrates relative to most other life. Arthropods and many other creatures have the brain and nerve cord running ventrally and the digestive tract running dorsally. Chordates have them reversed. It actually appears that the arthropod arrangement came first, and then somewhere millions of years ago the first chordates decided to swim on their backs instead. So your fly/human blend may be confused as to which way is up/down (or front/back if it’s standing upright).
Don’t make the mistake of thinking that spiders and insects are biologically similar even though they’re both arthropods. They’re as different from each other as we are from either of them.
Aren’t arthropods protostomes, while chordates are deuterostomes? So your hybrid might be confused about which orifice it should be speaking out of and which it should be shitting out of.
Indeed. This post was pretty thorough on the subject (scroll down a bit for the relevant parts):
[QUOTE=Blake]
How closely related arthropods are is highly unsettled. In fact there is still debate about whether arthropods even exist as a taxonomic group. It seems likely that the arthropods evolved multiple times from multiple ancestors and are not very closely related at all
The problem is that developmentally and physiologically arthropods are all very different. Far, far more different than any other phylum. Right from the very first cleavage patterns following fertilisation through to the way that teh eyes develop to the way that they manufacture their exoskeletons and the way that they move, the arthropods are more dissimilar to each other than they are to members of supposedly unrelated phyla. Moreover if they are part of the same phylum then multiple complex features must have evolved, then been lost, then re-evolved multiple times both within the phylum and within unrelated phyla. As you point out, in natural phyla you expect a majority of features to be consistent based upon a common ancestry. As far as I know the arthropods don’t share a single common feature beyond having segmented legs, and even their the joints differ radically. Even the presence of an exoskeleton is only common if you use a purely cosmetic definition of exoskeleton. If you apply a biochemical or developmental definition of what an exoskeleton is then many arthropods don’t have one.
At the moment the existence of any arthropod clade is in great dispute. It seems like the arthropods have evolved multiple times. The insects from an onychophoran ancestor, the crustaceans from an annelid ancestor and the arachnids from a nematode-like ancestor. They may look superficially similar because of the constraints of a rigid external skeleton, but developmentally and biochemically they don’t seem to have anything at all in common.
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Cave Johnson on combining preying mantis and human DNA.
I’m curious how you “average” an exoskeleton and an endoskeleton.
We could call them “politicians”.
If you want something human-sized, it’s going to have to rely more heavily on the human DNA to be functional. Bugs don’t have the skeleton, circulatory or respiratory sophistication to handle being more than a few ounces big. Their systems just don’t scale up. If you only have half a human’s lung capacity and half the bone density, your hybrid monster is going to be chasing people around in a wheelchair with an oxygen tank on the back. 
The same is probably true for most internal organs.
That means that playing around with the superficial things like number of limbs is about all you could do. In that regard, the Wikipedia article on the Hox genes may be of use - there’s a diagram there that shows which structures/genes are analogous between flies and people.
Bravo! You’ve managed to mention politics without even going off topic!
You probably end up with something with natural body armor, but that relies on the endoskeleton for movement & structural strength. Like an armadillo.
We’re supposed to have vestigial gills. It’s a series of looping capillaries on the sides of our necks. Those in insects are their wings.
I’m not sure that chordate gills have anything in common developmentally with insect gills; hence the OP question.