# What is the most effective location for spinnerets in a vertebrate?

In the large pillar forests of planet x, a group of vertebrates, in order to hunt down the fast climbing creatures that inhabit their habitat, rather than developing speed as well, went on another evolutionary path than its fellow vertebrates: they became ambush predators, making use of 2 (or more) spinneret organs to shoot silk at its prey, much like some gnaphosid spiders, with the main difference that their silk is adapted to temporarily affix prey to the surfaces they're clinging on. At times, this group cocoons prey as well, for storaging food during winter.

Now, what would be the most effective location for such spinnerets? If it weaved like a spider I'd go for the rear, but the fact that it uses them almost as guns tells me they should be somewhere closer to the head, maybe even inside the mouth. The closest fiction example I have is, well, Toby's version of spiderman. What would be the most efficient positioning? Could such organs develop naturally in a vertebrate (and not like the transgenic goat wigh spider milk type of developing) to begin with?

NOTE: the creatures are the size of an average Pitbull, are arboreal and quadrupedal.

• Hands seem like a terrible idea, given that it would take up room and reduce the possible dexterity of the limb, and thus decrease the effective use of the limb. – Halfthawed Mar 20 at 21:24

TL;DR - jaw muscles are likely required.

What distance does the jet need to travel and what nozzle speed does it need? Let's assume a 10m distance to the target (shorter than that and the pitbull-sized ambush predator is better off with a fang-and-claw physical attack) and we want the jet to reach the target in 250ms - longer than that and the prey has enough time to take a jump outside the targeted area. So, muzzle velocity at 40m/s.

We'll assume and ideal (non-viscous) sticky liquid and apply the Bernoulli law: $$\frac{p}{\rho} + \frac{v^2}{2} + g\cdot z = ct$$

Assume the speed of the sticky inside the reservoir at 0 (happens only if the cross-section of the reservoir is much larger than the muzzle) and out of the muzzle the liquid pressure drops to 0. We'll also assume a density of the liquid to be that of water 1000$$\frac{kg}{m^3}$$ and try to compute what pressure needs to be created inside the liquid reservoir to achieve that.

$$\frac{p_{res}}{\rho} = \frac{v^2_{muzzle}}{2} \xrightarrow{} p_{res} = 1000 \cdot \frac{40^2}{2} Pa \xrightarrow{} p_{res} = 800000 Pa = 7.895 atm$$ Probably we'll need to double that due to viscosity and the friction at the muzzle, but that's plausibly achievable by strong muscles.

This "strong muscles" requirement is reinforced by considerations on the power they need to deliver.

The energy consumption will depend on the amount of liquid projected. At a pitbull-size animal will weight about 15-35kg (let's take 25kg as the average) and will hunt prey in the 10-20% of his body weight - something like a turkey maybe? I can't see immobilizing such a prey with less than 0.5litres of goo.

I'll assume the total purge of the goo need to happen in another 200ms. The kinetic energy of projecting 0.5kg at 40m/s is 10kJ - which means a required power of 50kW. If you ride a gym bicycle, you'll know how it feels to generate 200W of power by the long muscles in you legs.

Fortunately for this case this power needs not to be sustainably delivered, but explosively. Nevertheless, the muscles should be strong and short to pull such feat in 0.2s.

One on top of the other, I would go for the jaw muscles - short and strong enough.

• I'm not sure if you mean that the mouth would be a good place or just described the kind of musculature necessary for it to work, but either way thank you for the insight – ProjectApex Mar 21 at 22:40
• @ProjectApex I mean put them somewhere the jaw muscle can act upon them - pouch squeeze or piston-like pressure. Doesn't need to be always in the firing position, an additional muscle can drag them there at the appropriate moment. Somewhere on the cheeks or under/alongside the tongue with the ejection orifice under the jaw or on the skull temples – Adrian Colomitchi Mar 21 at 23:19

Paired glands at the sides of the head.

1. These glands could rationally have evolved from salivary glands. Mammals have several types and can spare one to produce sticky goo. Goo would be kept in some reservoir prior to shooting.

2. Mammals aim using the eyes. Glands close to the eyes would follow a path from eyes to target.

3. Glands to the side of and under the eyes would not foul the eyes, ears, nose or mouth if they dribble.

• That would make for a singularly awful child rearing experience. Drool, snot, whatever they’ve smeared all over their face and when you try to wipe it up you’ve got to deal with dribbling spinnerettes as well. Eurgh. – Joe Bloggs Mar 21 at 15:16