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"Montaditos" is a form of "montadito", a noun which is often translated as "open sandwich". "Pulgas" is a form of "pulga", a noun which is often translated as "flea". Learn more about the difference between "montaditos" and "pulgas" below.
el montadito(
mohn
-
tah
-
dee
-
toh
)A masculine noun is used with masculine articles and adjectives (e.g., el hombre guapo, el sol amarillo).
a. open sandwich 
Ayer cené dos deliciosos montaditos de jamón serrano.I had two delicious open sandwiches topped with Serrano ham for dinner last night.
b. canapé  (on a slice of baguette)
Nos sirvieron unos montaditos variados con las bebidas.They served us a variety of canapés with the drinks.
la pulga(
pool
-
gah
)A feminine noun is almost always used with feminine articles and adjectives (e.g., la mujer bonita, la luna llena).
1. (insect)
a. flea 
Mi perro está lleno de pulgas y no para de rascarse.My dog is covered in fleas and cannot stop scratching himself.
a. small sandwich 
Pedí una pulga de chorizo en el restaurante de tapas.I ordered a small chorizo sandwich at the tapas restaurant.