Have

Devouring Swarm

Flying
Sacrifice a creature: Devouring Swarm gets +1/+1 until end of turn.

Grist, the Hunger Tide

As long as Grist, the Hunger Tide isn't on the battlefield, it's a 1/1 Insect creature in addition to its other types.
+1: Create a 1/1 black and green Insect creature token, then mill a card. If an Insect card was milled this way, put a loyalty counter on Grist and repeat this process.
−2: You may sacrifice a creature. When you do, destroy target creature or planeswalker.
−5: Each opponent loses life equal to the number of creature cards in your graveyard.

Conduit of Worlds

You may play lands from your graveyard.
{T}: Choose target nonland permanent card in your graveyard. If you haven't cast a spell this turn, you may cast that card. If you do, you can't cast additional spells this turn. Activate only as a sorcery.

Dread Return

Return target creature card from your graveyard to the battlefield.
Flashback—Sacrifice three creatures. (You may cast this card from your graveyard for its flashback cost. Then exile it.)

Durable Coilbug

{4}{B}: Return Durable Coilbug from your graveyard to your hand.

Fen Hauler

Improvise (Your artifacts can help cast this spell. Each artifact you tap after you're done activating mana abilities pays for {1}.)
Fen Hauler can't be blocked by artifact creatures.

Crawling Sensation

At the beginning of your upkeep, you may mill two cards.
Whenever one or more land cards are put into your graveyard from anywhere for the first time each turn, create a 1/1 green Insect creature token.

Kraul Swarm

Flying
{2}{B}, Discard a creature card: Return Kraul Swarm from your graveyard to your hand.

Necrosquito

Flying
Necrosquito enters the battlefield with two oil counters on it.
Necrosquito gets +1/+1 for each oil counter on it.
Whenever another creature or artifact you control is put into a graveyard from the battlefield, put an oil counter on Necrosquito.

Prakhata Pillar-Bug

{B}: Prakhata Pillar-Bug gains lifelink until end of turn. (Damage dealt by this creature also causes you to gain that much life.)

Getting

Hornet Queen

Flying, deathtouch
When Hornet Queen enters the battlefield, create four 1/1 green Insect creature tokens with flying and deathtouch.

Skittering Heartstopper

{B}: Skittering Heartstopper gains deathtouch until end of turn.

Giant Ankheg

Trample
Ward {2} (Whenever this creature becomes the target of a spell or ability an opponent controls, counter it unless that player pays {2}.)
Other creatures you control have trample and ward {2}.

Spinal Centipede

When Spinal Centipede dies, put a +1/+1 counter on target creature you control.

Halo Scarab

{2}, Exile Halo Scarab from your graveyard: Create a Treasure token. (It's an artifact with "{T}, Sacrifice this artifact: Add one mana of any color.")

Moldgraf Millipede

When Moldgraf Millipede enters the battlefield, mill three cards, then put a +1/+1 counter on Moldgraf Millipede for each creature card in your graveyard. (To mill a card, put the top card of your library into your graveyard.)

Masked Vandal

Changeling (This card is every creature type.)
When Masked Vandal enters the battlefield, you may exile a creature card from your graveyard. If you do, exile target artifact or enchantment an opponent controls.

Bilious Skulldweller

Deathtouch
Toxic 1 (Players dealt combat damage by this creature also get a poison counter.)

Giant Ladybug

Reach
When Giant Ladybug enters the battlefield, you may search your library for a basic land card, reveal it, then shuffle and put that card on top.

Giant Mantis

Reach (This creature can block creatures with flying.)

Duskshell Crawler

When Duskshell Crawler enters the battlefield, put a +1/+1 counter on target creature.
Each creature you control with a +1/+1 counter on it has trample.

Nessian Hornbeetle

At the beginning of combat on your turn, if you control another creature with power 4 or greater, put a +1/+1 counter on Nessian Hornbeetle.

Atraxa's Skitterfang

Atraxa's Skitterfang enters the battlefield with three oil counters on it.
At the beginning of combat on your turn, you may remove an oil counter from Atraxa's Skitterfang. When you do, target creature you control gains your choice of flying, vigilance, deathtouch, or lifelink until end of turn.

Adaptive Shimmerer

Flash
Adaptive Shimmerer enters the battlefield with three +1/+1 counters on it.

Loam Larva

When Loam Larva enters the battlefield, you may search your library for a basic land card, reveal it, then shuffle and put that card on top.

It of the Horrid Swarm

Emerge {6}{G} (You may cast this spell by sacrificing a creature and paying the emerge cost reduced by that creature's mana value.)
When you cast this spell, create two 1/1 green Insect creature tokens.

Symbiotic Beast

When Symbiotic Beast dies, create four 1/1 green Insect creature tokens.

Don't Have

Scute Swarm

Landfall — Whenever a land enters the battlefield under your control, create a 1/1 green Insect creature token. If you control six or more lands, create a token that's a copy of Scute Swarm instead.

Pestilent Syphoner

Flying
Toxic 1 (Players dealt combat damage by this creature also get a poison counter.)

Battlefly Swarm

Flying
{B}: Battlefly Swarm gains deathtouch until end of turn.