Having five or more eyes, Arachnidoids absorb more light from the enviroment, and can see in the dark.

 Arachnidoids' sizable abdomen closely resembles that of a spider. It can produce various types of webbing, intricately mixing between thickness (1mm-0.1mm), stickiness (like tape, up to flypaper levels of adhesion), and length (a thicker/stickier/longer strand will take longer to make).

The webs are white, but can be dyed. Their overall properties are similar to rope, but their strength is four times higher than steel (by area). They are still flammable however, and will burn a lot like flesh if not doused.

Clothing can be fashioned out of the webs, but creating durable and protective armor requires a sophisticated treatment process to protect the material from flame and high heat, and blow resistant plating weaved into it, which would require additional materials and special techniques. A fire dragon's scales and blood would likely complement them well.

Arachnidoids' fangs are sharp, and can be used to inject venom.

You can make a bite attack against a creature within 5ft that you have grappled or you have advantage on melee attacks on. It uses your unarmed strike for to-hit and damage bonuses, but deals 1d6 piercing damage. Additionally, an amount of times equal to your proficiency bonus, you can inject venom into a target hit by this attack. The creature makes a constitution saving throw with a DC of 8 + your proficiency modifier. On a failed save, the creature takes 1d8 poison damage and is poisoned for 1 minute, but if they are already poisoned by this attack they are also incapacitated, and the duration resets. On a successful save they take half the poison damage and aren't poisoned. The creature can repeat the saving throw at the end of their turn.

Arachnidoids have six arms, and can wield more equipment. This contributes to their overall athleticism.

Using more than two weapons at a time, or trying to poke out of a shield with more than one is still impractical, but holding items for quickly drawing them or dragging bodies around could still be viable. 

The prices and availability of armors may worsen as a result.

Arachnidoid's claws, albeit small, are quite useful. They easly hook onto things and provide excellent grip letting Arachnidoids climb easly, and giving them proficiency in athletics.

Arachnidoids' musculatory systems are weaker than with most humanoids, but are supplemented by the hydraulic properties of their cardiovascular system. This doesn't help in fighting, as muscles are still relied on for fine movements and jolts of speed, but this hydraulic system adjusts to a larger load effortlessly, doubling Arachnidoids' carrying capacity.

Arachnidoids attune to their spider likeness to amplify their abilities.

As an arachnidoid, you can cast either the Web or Spider Climb spell, up to an amount of times equal to your proficiency bonus (with a shared count) per long rest.
The spellcasting ability for these spells is constitution.

When determining your character’s ability scores, increase one score by 2 and increase a different score by 1, or increase three different scores by 1. Follow this rule regardless of the method you use to determine the scores, such as rolling or point buy. You can’t raise any of your scores above 20.

In accordance with TCE.

You pick up on the language you grew around in. Choose common or a different language based on the setting.