When referring to an animal of unknown sex, I try to default to female pronouns. It's only fair, male pronouns have been the default since--I don't know, all of recorded history I suppose, at least in the West. There's some biological merit to it, as well. Females are the norm, males are an aberrant version of the female concocted to provide genetic variation.
With spiders, males are distinctive enough that we bug people will often lead with the fact of their sex. "Well, first of all it's a male." Small relative size, slim not plump abdomen, and usually visibly large palps. Palps are leg-like appendages on the front end of the spider, used by the males to transfer sperm from their genitalia to the female's. I don't see them on this little spider, but I do see comically large front legs. One commenter already remarked that it looked like he skipped his lower body day at the gym.
I suspect that these front legs are used by this jumping spider Tutelina harti
* as visual communicators. Many male salticids** wave their front legs in various distinct patterns. Sometimes they do it to communicate with other males, probably territorial messages. More importantly, they signal to the female, to indicate that they are males of the same species, to convince the larger more powerful female spider not to eat them.
Bugguide says that these spiders are "usually found on tree bark
." I guess the fabric of my shorts was close enough.
* Tutelina was a Roman goddess who protected the crops. Harti refers to a man named Hart (dunno who). A hart is another word for a deer.
** "Family of dancers."