Male spiders eject semen from their genitalia but deliver it with their palps, at no small risk. So seeing the white goo below her abdomen would imply arachnid futa; the absence of enlarged magical pedipalps around her maw would negate it.
you know, all things considered, this doesn't really look like rape. Sure, the girl a bit distressed but it seems more of a bit too much being put in.
The monster girl doesn't seem to be malicious or even have a classic rape face, and doesn't seem to be even trying to restrain the other girl. More like keeping balance.
The spider girl looks more miffed than anything. As if she was walking along, minding her own business, and noticed an elf lying around: "Oh damn it, not another lazy elf! Now I have to fuck her." Like it's a compulsion or something, hehe.
