Black Recluse

Type	Enemy
Environment	Spider Cave
AI Type	Fighter AI (ground); Spider AI (on wall)
Damage	90/180/270 (ground); 100/200/300 (on wall); 36 (web spit)
Max Life	350/700/1050
Defense	40
KB Resist	75%/78%/80%
Banner	Black Recluse Banner
Immune to
Debuff	Acid Venom
Debuff tooltip	Losing life
Chance	10%
Duration	4 seconds / 8 seconds8 seconds10 seconds
Debuff	Webbed
Debuff tooltip	You are stuck
Chance	100% (web spit)
Duration	0.5–2.5 seconds
Coins	51250
Hurt
Killed
Web spit

The Black Recluse is a Hardmode enemy that spawns in Spider Caves, but can rarely be found in the Cavern layer outside of its caves. It can inflict the Acid Venom debuff. In Expert Mode, it gains a special spit attack that applies the Webbed debuff to players.

Notes[]

Like all spiders, Black Recluses cannot travel through a vertical 2-tile wide shaft.
The cooldown between web spit attacks lasts 3 seconds. It is paused if the Black Recluse loses focus on the player or has no direct line of fire, and it is increased by between 0.33 and 1 second when the Black Recluse takes damage.^[1]
Black Recluses completely replace Wall Creepers in the Spider Caves once Hardmode starts. On PC, Console, Mobile, and tModLoader, Wall Creepers won't spawn in Hardmode.

With their black color, Black Recluses may be difficult to perceive in the Spider Cave. It is advisable to place an ample number of light sources or drink a Hunter Potion.
Their high damage output combined with their property of being difficult to see and chance to inflict the Venom debuff for an additional 40 total damage makes them extremely deadly and can easily kill players in early Hardmode.
Because of their low health and high defense, using weapons that deal a lot of damage with a single strike is the best way of defeating them. High-knockback weapons can also hold them off for long enough to get them killed.
- A good weapon to use on a Black Recluse is a yoyo, as it can deal damage to it from a safe distance.
Once the player has better weapons, Black Recluses are an excellent money-farming target, offering over half a Gold Coin per kill. They drop 5 up front, the chance of a Poison Staff averages about 1150 per kill, and the fangs can be crafted into Queen Spider Staffs averaging 2396 per kill.

The name Black Recluse is a combination of the spider names black widow and brown recluse. Both spiders are commonly known for their venomous capabilities. This is reflected in the enemy's appearance, drops, and Venom debuff infliction.
While the anatomy is accurate it is missing pedipalps, this is also applicable to the Wall creepers.
The cooldown of the web spit attack is programmed in a way that it could technically last up to 15 seconds:^[1] Every tick^[2], the game checks whether the last attack occurred longer than x ticks ago, x being a number between 180 and 899 randomly picked upon every check. If that is the case, the cooldown is reset and the Black Recluse fires again. Therefore, the cooldown will always last at least 181 ticks (which is equal to 3 seconds):

Ticks	Chance ${\color{white}\frac{719!\cdot(k+1)}{(719-k)! \cdot 720^{k+1}}}$
181	0.138889%
182	0.277391%
183	0.550938%
184	0.684846%
185	0.816108%
207	2.28911%
229	1.27947%
230	1.21673%
231	1.15488%
232	1.09411%
233	1.03461%
281	0.00888%

It can be seen that the chances for the cooldown time increase steadily before hitting a maximum and decreasing at a similar rate. Note that after 281 ticks, chances become negligible.
The chance that the cooldown lasts 899 ticks, or 15 seconds, is so unimaginably low (on the order of 10^-690) that it is virtually zero.

The Bestiary entry for the Black Recluse: "They have eight legs, eight eyes, and eight ways to cocoon and feast upon an unsuspecting victim."

Desktop 1.4.0.1:
- Now drops the Fried Egg.
- Reduced the duration of Acid Venom from 8 to 4 seconds.

↑ ^1.0 ^1.1 Information taken from the PC 1.4.0.5 source code, method AI() in Terraria.NPC.cs. There may be inaccuracies, as the current PC version is 1.4.4.9.
↑ A tick is a time unit countable by the software. Most of Terraria's updating logic happens every tick. A tick has the length of 1/60th of a second, hence there are 60 ticks in a second and 3600 ticks in a minute.