What are arachnid insects?

There's no such thing as 'arachnid insects'. The phrase is a contradiction in terms. The Word 'arachnid' refers to membership in the class Arachnida. Such members are recognized by their lack of antennae or wings, hard skeleton that's found on the outside of their body parts, and four sets of jointed legs that they tend to have as adults. Examples include such non insects as water boatmen, ticks, Spiders, scorpions, mites, and harvesters.

The term 'insects' refers to membership in the class Insecta. Such members are recognized by their two antennae, three sets of jointed legs, three segmented bodies, and hard skeleton that's found on the outside of their body parts.