Venus flytraps very commonly digest things like laybirds and beetles; it will never cause the trap to die, but it just takes the trap longer to digest beetle like insects rather than something like a fly. In the end after digesting, the hard exoskeleton of the insect is all thats left since the flytrap doesnt digest that. Traps also wont die due to excessive sunlight; quite the opposite actually. Venus flytraps love twelve hours of direct sunlight; the more sunlight, the healthier the plant.
Your plant looks quite healthy actually, I wouldn't really worry if i were you. But make sure you give it lots and lots of direct sunlight and leave it in a tray of distilled water, but not in winter, as the plant doesnt need as much water while its dormant; just make sure the surface of the soil looks moist. And no need to feed, they've adapted to catch their own insects themselves and survive on minimal nutrients
By the way, the brown marks mean that that particular trap will rot and eventually blacken. But dont worry, its normal for traps to rot and die back, but as one dies, another few will grow in its place, so it'll always increase in number with the right conditions
But if you want to know what usually causes the traps to rot, keep reading below.
Traps usually blacken due to two reasons. One being age, the most common cause, as a trap usually only lasts after digesting prey 1 - 4 times, therefore having a limited lifespan. The second being an individual trap can also rot if the prey its caught is too big for digestion; the proper sized prey for a trap to properly digest is one third of the traps size at most.
A third and more uncommon cause from my personal experiences is due to the venus flytrap's ability to digest things. I say this, because a venus flytrap receiving heaps of sunlight everyday (8 hours for example) can digest a few insects at a time, with one insect per trap, having no problem. But a venus flytrap receiving less sunlight will not be able to, and if a few traps on a sunlight deprived plant catch an insect each, for example, their ability to metabolise wont be able to keep up with the prey quantity, therefore causing traps to rot and blacken overtime.
Hope that helped! Greetings from Australia
By the way, the brown marks mean that that particular trap will rot and eventually blacken. But dont worry, its normal for traps to rot and die back, but as one dies, another few will grow in its place, so it'll always increase in number with the right conditions 
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