Some nymphs have fairly thick cuticles. Most immatures are preserved in ethanol or other suitable media. Ethanol tends to make specimens brittle. One notable exception is some places will preserve caterpillars by freeze drying or replacing the internal tissues with cotton.

In my opinion, maintenance of dry material is easier. However, I have generally had a large collection of dry material with a few specimens in liquid. There may be more efficient methods of preservation in liquid that I have not experienced.

Arachnids, myriapods and crustaceans are not insects

Imagine you want to compare 200 insect species in a given family. If your material is preserved in ethanol, you have 200 vials to open/close before you can examine them with your binocular microscope. With dry material, you just open your entomological box and there you go !

In other words, why would entomologists use complicated and expensive methods when it can be easier and cheaper ?

arachnids and crustacean and insects those are classes of arthropods and they differ from each other.

ethanol is used in preservation of insect , mostly 70% ethanol.

I would guess spiders are preserved in ethanol because dried specimens end up with too many missing legs. Some insect groups have this problem. The Tipulidae (Diptera) is one group, but these are still dry mounted.

I think it is difficult to preserve soft bodied insect in dry condition. Hence, 70% ethanol is used. For large and hard bodied insect, we can directly go for dry preservation. And if we are going for DNA extraction, then in that case preservation in 70% ethanol or other suitable media is good for both hard bodied and soft bodied arthropods.

To Ricardo Castro's answer, I comment that while myriapods and arachnids (both of the clade Paradoxopoda) are not insects, insects are, in a sense, crustaceans (clade Pancrustacea).

Soft bodied insects become distorted upon drying. The anatomical features used in identification are hard to see in a dried soft bodied specimen and the specimen needs to be rehydrated. In a hard bodied specimen most of the relevant features are still obvious after drying with little or no loss of detail.