I'll let the experts comment but I came back last week. We ended up wearing long trousers more than shorts just because the sun was so strong (and we're not used to it!) but either would have been fine. Occasionally hikes started in a cooler wetter Highlands before moving to a hotter drier area e.g the Sierra Negra hike on Isabela. Here long trousers were preferable.
It should of course be slightly cooler when you're there. When we were there the temperature was early 20s (centigrade) but the strong sun made it feel much hotter.
We didn't have any problems with mosquitos but on several beaches horseflies were common & bit quite a few people. My partner got quite a few bites - once even through a shirt sleeve. I did use repellent but they seemed to bite around the waters edge & one guide said they especially like wet skin..