British chevrons

I came across these two photos, the first one is of a warrant officer first class in No. 15 Squadron (R.E.8) during a debrief on 25th March 1918. The second is of, oddly enough, an observer who was photographed with the pilots of No. 1 Squadron (S.E.5a) at Clairmarais on 3rd July 1918. Perhaps he was one of the staff officers, who knows?
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But here is my question. On the cuff of the right sleeve of each are small chevrons. The warrant officer has two of them (very easily seen in the inset) while the observer seems to have three of them (the inset for him is not as well defined). What are they for? Are they length of service stripes? Overseas stripes? Or are they the "wound stripes" of which I have heard, but have never seen an example? If they're not wound stripes, can someone tell me what those looked like and where they were worn on the uniform?
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Thanks in advance for any help anyone can offer.
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You are almost right; they are War Service chevrons. You got a chevron for each year served during the war. I think they are red for 1915-18 and there was a special blue one for service in 1914. They were only in effect for the duration of the war so the maximum you would see would be 5. I believe they may have come into effect in 1918 or so but will stand correction on that.

As far as Wound Stripes go they were a 2in (5cm) length of "Russian" Gold Braid. I would guess they would be only about a 1/4in wide. They were worn on the left sleeve just above the cuff. They were oriented vertically on the sleeve. They were awarded for each wound (I believe there was at least one soldier in the Australian Imperial Force who had seven!)

Thanks. I was hoping someone would know. The pictures were very confusing to me. I appreciate the info.