Chiggers are the worst. Or is it No See Ums? Or is it those brown recluse spiders? I hate them all.
I remember going through SOI at Camp Pendleton spending the night in a hole we dug just before sundown. Tarantulas. Lots of Tarantulas. Hand to hand combat all night. Glad to report we won but it wasn't your typical training exercise.
I have yet to meet one person, in all the years I lived in tarantula territory, that was ever seriously injured by one. Or even moderately injured. I've had them walk up and inspect my bare toe while I was wearing sandals, tap tap tap with their front legs, then turn away and drop into a pencil diameter hole.
Lying there at Pendelton, the least of my worries would be tarantulas. Scorpions, black widows, rattlers, rat mites, fleas, and poison oak would be way more of a concern depending upon where your exercises were held.
