what is the anatomical modifications that have been important for insects to thrive in a terrestial habitat?

Question

Also: What are the evolutionary trends seen in insects within the neoptera? What is the comparison between diptera and lepidoptera? and what is the differences and similarities between arachnida and hexapoda? Ectoparasitic insects what are they and what about them?

Answer 1

Hmmm...wow...a VERY general question. This is out of my field, but your other questions were so engaging I decided to see what else you have asked...

For the first one (how do they thrive/what modifications do they have). I am going to list things insects all have in common and that kill them when things go wrong with these systems:
Q1:
1) A chitinous exoskeleton, usually with a waxy covering that conserves water and provides mechanical protection.
2) A system of spicules that enables oxygen/CO2 exchange without having to resort to an overly flexible out membrane keeping that mechanical protection sound.
3) Segmented bodies and legs to allow greater flexibility of movement than is achievable solely with a segmented exoskeleton
4) Wings for flight or additional mechanical protection
5) Rudimentary circulatory system (this one might not qualify since I bet their ancestors had something like this too.)

Q2:
There are a LOT of kinds of neoptera and I don't know much evolutionary biology so I will not attempt that one. Probably one trend has to do with the falling partial pressure of oxygen over the past few hundred million years and the guys getting smaller to cope. They all have flexible wings. I dunno what else. Sorry.

Q3: Compare flies to butterflies/moths/skippers.
Commonalities: all 4 wings have flight function; larval stage that pupates; adults have tubular mouth parts
Differences: antenna, scaled wings.
That is a start anyway.

Q4: Similarities: Chitinous exoskeletons, segmented bodies (but different numbers of segments), jointed legs,
Differences: wings vs. more legs, compound vs. simple eyes, pupation/metamorphosis vs. nymphal stages

Q5: Ecto means outside, and everyone know what parasitic means, so this would include lice, fleas, maybe mosquitos (they only suck blood for egg laying, you know), certain kinds of flies and true bugs. They mostly live on blood, but I think some eat dead skin. Geez, what else CAN you eat living on the outside of a critter?

Whew. Tough.