добавь в конец
.menu_item:hover:after, .menu_item:hover:before{
display:block;
}
.menu_item{
position: relative;
}
.menu_item:after{
position: absolute;
content: " ";
display: none;
width: 1px;
height: 1px;
left: -10px;
border-left: 5px solid transparent;
border-right: 5px solid transparent;
border-bottom: 23px solid #3a5362;
transform: rotate(-86deg) scale(.5);
bottom: -10px;
}
.menu_item:before{
position: absolute;
content: " ";
display: none;
width: 1px;
height: 1px;
border-left: 5px solid transparent;
border-right: 5px solid transparent;
border-bottom: 23px solid #3a5362;
right: -10px;
transform: rotate(86deg) scale(.5);
bottom: -10px;
}