Picar vs. morder
When talking about insect bites and stings in Spanish, the choice between the verbs picar and morder depends on the anatomy of the creature and how it attacks. While both can cause pain, they describe entirely different physical actions.
Picar
The verb picar means to sting, prick, or puncture. It is used when an insect or animal pierces the skin using a stinger, a proboscis, or fangs. You should use picar for creatures like mosquitoes, bees, wasps, scorpions, and spiders.
Me picó un mosquito en el brazo mientras dormía.
(A mosquito bit me on the arm while I was sleeping.)
Ten cuidado con esa avispa porque te puede picar.
(Be careful with that wasp because it can sting you.)
Si te pica una abeja, debes extraer el aguijón rápidamente.
(If a bee stings you, you must remove the stinger quickly.)
Dicen que las arañas de esta región no suelen picar a los humanos.
(They say that the spiders in this region do not usually bite humans.)
El escorpión intentó picar al perro en el jardín.
(The scorpion tried to sting the dog in the garden.)
Siempre me pongo repelente para que no me piquen los bichos en el bosque.
(I always put on repellent so that the bugs do not bite me in the forest.)
Morder
The verb morder translates to biting with teeth or jaws. In the insect world, you use morder for bugs that possess mandibles and physically clamp down on your skin, rather than stinging or piercing it. It is commonly used for ants, beetles, and larger creatures.
Me senté en el pasto y una hormiga me empezó a morder.
(I sat on the grass and an ant started to bite me.)
Ese escarabajo grande puede morder si lo atrapas con la mano cerrada.
(That big beetle can bite if you catch it with your closed hand.)
Las hormigas rojas muerden con mucha fuerza cuando defienden su nido.
(Red ants bite with a lot of force when they defend their nest.)
Ayer vi a un ciempiés morder a una presa más pequeña entre las rocas.
(Yesterday I saw a centipede bite a smaller prey among the rocks.)
Aunque es un insecto diminuto, tiene mandíbulas lo suficientemente fuertes para morder.
(Even though it is a tiny insect, it has jaws strong enough to bite.)
Summary
To choose the right word, think about the insect's anatomy. If the insect punctures the skin with a stinger or needle-like mouthpart, use picar. If the insect uses jaws or mandibles to take a physical bite, use morder.